Revert "Fix incorrect 'unreachable code' error in SkSL"

This reverts commit 67d2d73d06.

Reason for revert: Triggers errors in some shaders.

Original change's description:
> Fix incorrect 'unreachable code' error in SkSL
>
> This trades one error for another (a potential for incorrect use of
> unassigned variables). False-positives for unassigned variables are
> straightforward to workaround (and produce code that still looks
> reasonable). Working around unreachable code errors is tricky, and
> likely to produce non-idiomatic code. This change also makes the data
> flow analysis of all loop constructs more similar - for loops were
> behaving very differently from while loops.
>
> Note that this effectively a revert of:
>   https://skia-review.googlesource.com/c/skia/+/18121/
>
> Change-Id: Ib85d90b22cac8addfb106459c0a5f5616a89c3eb
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/344957
> Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
> Commit-Queue: Brian Osman <brianosman@google.com>

TBR=brianosman@google.com,ethannicholas@google.com,johnstiles@google.com

Change-Id: I84b93cc7f9309446dcc0e949e90908df31a1ff9c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/345119
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>

src/sksl/SkSLCFGGenerator.cpp

@@ -602,8 +602,13 @@ void CFGGenerator::addStatement(CFG& cfg, std::unique_ptr<Statement>* s) {
             fLoopExits.push(loopExit);
             if (f.test()) {
                 this->addExpression(cfg, &f.test(), /*constantPropagate=*/true);
-                BlockId test = cfg.fCurrent;
-                cfg.addExit(test, loopExit);
+                // this isn't quite right; we should have an exit from here to the loop exit, and
+                // remove the exit from the loop body to the loop exit. Structuring it like this
+                // forces the optimizer to believe that the loop body is always executed at least
+                // once. While not strictly correct, this avoids incorrect "variable not assigned"
+                // errors on variables which are assigned within the loop. The correct solution to
+                // this is to analyze the loop to see whether or not at least one iteration is
+                // guaranteed to happen, but for the time being we take the easy way out.
             }
             cfg.newBlock();
             this->addStatement(cfg, &f.statement());
@@ -613,6 +618,7 @@ void CFGGenerator::addStatement(CFG& cfg, std::unique_ptr<Statement>* s) {
                 this->addExpression(cfg, &f.next(), /*constantPropagate=*/true);
             }
             cfg.addExit(cfg.fCurrent, loopStart);
+            cfg.addExit(cfg.fCurrent, loopExit);
             fLoopContinues.pop();
             fLoopExits.pop();
             cfg.fCurrent = loopExit;

tests/sksl/errors/Unreachable.sksl

@@ -3,4 +3,5 @@ void call_after_return()                  { return; call_after_return(); }
 void code_after_continue()                { for (;;) { continue; int x = 1; } }
 void code_after_if_else_both_sided_exit() { if (true) return; else discard; return; }
 
+// Not detected today
 void code_after_infinite_loop()           { for (;;) {} return; }

tests/sksl/errors/golden/Unreachable.glsl

@@ -4,5 +4,4 @@ error: 1: unreachable
 error: 2: unreachable
 error: 3: unreachable
 error: 4: unreachable
-error: 6: unreachable
-5 errors
+4 errors