Fix variable scope when switch block exits multiple times.

Inner scope can still dominate here, so we need to be conservative when
we observe switch blocks specifically. Normal selection merges cannot
merge from multiple paths.

reference/shaders-no-opt/asm/frag/switch-single-case-multiple-exit-cfg.asm.frag

@@ -0,0 +1,29 @@
+#version 310 es
+precision mediump float;
+precision highp int;
+
+layout(location = 0) out highp vec4 _GLF_color;
+
+vec2 _19;
+
+void main()
+{
+    highp vec2 _30;
+    switch (0)
+    {
+        default:
+        {
+            if (gl_FragCoord.x != gl_FragCoord.x)
+            {
+                _30 = _19;
+                break;
+            }
+            highp vec2 _29 = _19;
+            _29.y = _19.y;
+            _30 = _29;
+            break;
+        }
+    }
+    _GLF_color = vec4(_30, 1.0, 1.0);
+}
+

shaders-no-opt/asm/frag/switch-single-case-multiple-exit-cfg.asm.frag

@@ -0,0 +1,57 @@
+; SPIR-V
+; Version: 1.0
+; Generator: Khronos Glslang Reference Front End; 7
+; Bound: 54
+; Schema: 0
+               OpCapability Shader
+          %1 = OpExtInstImport "GLSL.std.450"
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint Fragment %main "main" %gl_FragCoord %_GLF_color
+               OpExecutionMode %main OriginUpperLeft
+               OpSource ESSL 310
+               OpName %main "main"
+               OpName %gl_FragCoord "gl_FragCoord"
+               OpName %_GLF_color "_GLF_color"
+               OpDecorate %gl_FragCoord BuiltIn FragCoord
+               OpDecorate %_GLF_color Location 0
+       %void = OpTypeVoid
+          %3 = OpTypeFunction %void
+        %int = OpTypeInt 32 1
+      %int_0 = OpConstant %int 0
+      %float = OpTypeFloat 32
+    %v4float = OpTypeVector %float 4
+%_ptr_Input_v4float = OpTypePointer Input %v4float
+%gl_FragCoord = OpVariable %_ptr_Input_v4float Input
+       %uint = OpTypeInt 32 0
+     %uint_0 = OpConstant %uint 0
+%_ptr_Input_float = OpTypePointer Input %float
+       %bool = OpTypeBool
+    %v2float = OpTypeVector %float 2
+%_ptr_Output_v4float = OpTypePointer Output %v4float
+ %_GLF_color = OpVariable %_ptr_Output_v4float Output
+    %float_1 = OpConstant %float 1
+         %52 = OpUndef %v2float
+       %main = OpFunction %void None %3
+          %5 = OpLabel
+               OpSelectionMerge %9 None
+               OpSwitch %int_0 %8
+          %8 = OpLabel
+         %17 = OpAccessChain %_ptr_Input_float %gl_FragCoord %uint_0
+         %18 = OpLoad %float %17
+         %22 = OpFOrdNotEqual %bool %18 %18
+               OpSelectionMerge %24 None
+               OpBranchConditional %22 %23 %24
+         %23 = OpLabel
+               OpBranch %9
+         %24 = OpLabel
+         %33 = OpCompositeExtract %float %52 1
+         %51 = OpCompositeInsert %v2float %33 %52 1
+               OpBranch %9
+          %9 = OpLabel
+         %53 = OpPhi %v2float %52 %23 %51 %24
+         %42 = OpCompositeExtract %float %53 0
+         %43 = OpCompositeExtract %float %53 1
+         %48 = OpCompositeConstruct %v4float %42 %43 %float_1 %float_1
+               OpStore %_GLF_color %48
+               OpReturn
+               OpFunctionEnd

spirv_cfg.cpp

@@ -148,14 +148,35 @@ bool CFG::post_order_visit(uint32_t block_id)
 		// Add a fake branch so any dominator in either the if (), or else () block, or a lone case statement
 		// will be hoisted out to outside the selection merge.
 		// If size > 1, the variable will be automatically hoisted, so we should not mess with it.
+		// The exception here is switch blocks, where we can have multiple edges to merge block,
+		// all coming from same scope, so be more conservative in this case.
 		// Adding fake branches unconditionally breaks parameter preservation analysis,
 		// which looks at how variables are accessed through the CFG.
 		auto pred_itr = preceding_edges.find(block.next_block);
 		if (pred_itr != end(preceding_edges))
 		{
 			auto &pred = pred_itr->second;
-			if (pred.size() == 1 && *pred.begin() != block_id)
-				add_branch(block_id, block.next_block);
+			auto succ_itr = succeeding_edges.find(block_id);
+			uint32_t num_succeeding_edges = 0;
+			if (succ_itr != end(succeeding_edges))
+				num_succeeding_edges = succ_itr->second.size();
+
+			if (block.terminator == SPIRBlock::MultiSelect && num_succeeding_edges == 1)
+			{
+				// Multiple branches can come from the same scope due to "break;", so we need to assume that all branches
+				// come from same case scope in worst case, even if there are multiple preceding edges.
+				// If we have more than one succeeding edge from the block header, it should be impossible
+				// to have a dominator be inside the block.
+				// Only case this can go wrong is if we have 2 or more edges from block header and
+				// 2 or more edges to merge block, and still have dominator be inside a case label.
+				if (!pred.empty())
+					add_branch(block_id, block.next_block);
+			}
+			else
+			{
+				if (pred.size() == 1 && *pred.begin() != block_id)
+					add_branch(block_id, block.next_block);
+			}
 		}
 		else
 		{