[wasm] Fix set_local appearing in unreachable code.

R=ahaas@chromium.org
LOG=Y
BUG=chromium:575861

Review URL: https://codereview.chromium.org/1573053003

Cr-Commit-Position: refs/heads/master@{#33213}

src/wasm/ast-decoder.cc

@@ -903,7 +903,7 @@ class LR_WasmDecoder : public Decoder {
         LocalType type = LocalOperand(p->pc(), &index, &unused);
         Tree* val = p->last();
         if (type == val->type) {
-          if (builder_) ssa_env_->locals[index] = val->node;
+          if (build()) ssa_env_->locals[index] = val->node;
           p->tree->node = val->node;
         } else {
           error(p->pc(), val->pc, "Typecheck failed in SetLocal");

test/cctest/wasm/test-run-wasm.cc

@@ -1684,6 +1684,15 @@ TEST(Run_Wasm_Block_If_P_assign) {
 }
 
 
+TEST(Run_Wasm_DanglingAssign) {
+  WasmRunner<int32_t> r(MachineType::Int32());
+  // { return 0; p0 = 0; }
+  BUILD(r,
+        WASM_BLOCK(2, WASM_RETURN(WASM_I8(99)), WASM_SET_LOCAL(0, WASM_ZERO)));
+  CHECK_EQ(99, r.Call(1));
+}
+
+
 TEST(Run_Wasm_ExprIf_P) {
   WasmRunner<int32_t> r(MachineType::Int32());
   // p0 ? 11 : 22;