tint/transform/builtin_polyfill: Don't polyfill @const builtins

We don't want to replace builtins that are constant-expression
evaluated, as the replacement cannot be used as a constant expression.

Fixed: tint:1667
Change-Id: I554d9884fc41890247ee64b47a70621be5fcdbe5
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/107680
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Auto-Submit: Ben Clayton <bclayton@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

src/tint/transform/builtin_polyfill.cc

@@ -569,6 +569,9 @@ bool BuiltinPolyfill::ShouldRun(const Program* program, const DataMap& data) con
         for (auto* node : program->ASTNodes().Objects()) {
             if (auto* call = sem.Get<sem::Call>(node)) {
                 if (auto* builtin = call->Target()->As<sem::Builtin>()) {
+                    if (call->Stage() == sem::EvaluationStage::kConstant) {
+                        continue;  // Don't polyfill @const expressions
+                    }
                     switch (builtin->Type()) {
                         case sem::BuiltinType::kAcosh:
                             if (builtins.acosh != Level::kNone) {
@@ -653,6 +656,9 @@ void BuiltinPolyfill::Run(CloneContext& ctx, const DataMap& data, DataMap&) cons
         State s{ctx, builtins};
         if (auto* call = s.sem.Get<sem::Call>(expr)) {
             if (auto* builtin = call->Target()->As<sem::Builtin>()) {
+                if (call->Stage() == sem::EvaluationStage::kConstant) {
+                    return nullptr;  // Don't polyfill @const expressions
+                }
                 Symbol polyfill;
                 switch (builtin->Type()) {
                     case sem::BuiltinType::kAcosh: