Only check def dominance of reachable uses

Fixes https://github.com/KhronosGroup/SPIRV-Tools/issues/295

CHANGES

@@ -13,6 +13,7 @@ v2016.2-dev 2016-07-19
    #279: validator: infinite loop when analyzing some degenerate control
      flow graphs
    #290: disassembler: never generate bare % for an identifier
+   #295: validator: def-use dominance check should ignore unreachable uses
 
 v2016.1 2016-07-19
  - Fix https://github.com/KhronosGroup/SPIRV-Tools/issues/261

source/validate_id.cpp

@@ -2392,6 +2392,7 @@ spv_result_t CheckIdDefinitionDominateUse(const ValidationState_t& _) {
         // If the Id is defined within a block then make sure all references to
         // that Id appear in a blocks that are dominated by the defining block
         for (auto use : definition.second.uses()) {
+          if (!use->reachable()) continue;
           if (use->dom_end() == find(use->dom_begin(), use->dom_end(), block)) {
             return _.diag(SPV_ERROR_INVALID_ID)
                    << "ID " << _.getIdName(definition.first)

test/Validate.SSA.cpp

@@ -1234,6 +1234,44 @@ TEST_F(ValidateSSA, DISABLED_PhiVariableDefMustComeFromBlockDominatingThePredece
   // TODO(dneto): Check for a good error message
 }
 
+TEST_F(ValidateSSA, DominanceCheckIgnoresUsesInUnreachableBlocksDefInBlockGood) {
+  string str = kHeader
+      + kBasicTypes +
+               R"(
+%func        = OpFunction %voidt None %vfunct
+%entry       = OpLabel
+%def         = OpCopyObject %boolt %false
+               OpReturn
+
+%unreach     = OpLabel
+%use         = OpCopyObject %boolt %def
+               OpReturn
+               OpFunctionEnd
+)";
+
+  CompileSuccessfully(str);
+  EXPECT_EQ(SPV_SUCCESS, ValidateInstructions()) << getDiagnosticString();
+}
+
+TEST_F(ValidateSSA, DominanceCheckIgnoresUsesInUnreachableBlocksDefIsParamGood) {
+  string str = kHeader + kBasicTypes +
+               R"(
+%void_fn_int = OpTypeFunction %voidt %intt
+%func        = OpFunction %voidt None %void_fn_int
+%int_param   = OpFunctionParameter %intt
+%entry       = OpLabel
+               OpReturn
+
+%unreach     = OpLabel
+%use         = OpCopyObject %intt %int_param
+               OpReturn
+               OpFunctionEnd
+)";
+
+  CompileSuccessfully(str);
+  EXPECT_EQ(SPV_SUCCESS, ValidateInstructions()) << getDiagnosticString();
+}
+
 TEST_F(ValidateSSA, UseFunctionParameterFromOtherFunctionBad) {
   string str = kHeader +
                "OpName %first \"first\"\n"