[wasm] Remove reachability check in graph building

The interface will now only be called for reachable code, hence the
check in the graph builder is not needed any more. We DCHECK instead.

R=titzer@chromium.org

Change-Id: I24ef96d62d6a5cda18d9efefcbd089d5f324f624
Reviewed-on: https://chromium-review.googlesource.com/716176Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48675}

src/wasm/function-body-decoder.cc

@@ -60,12 +60,12 @@ struct SsaEnv {
   }
 };
 
-// Macros that build nodes only if there is a graph and the current SSA
-// environment is reachable from start. This avoids problems with malformed
-// TF graphs when decoding inputs that have unreachable code.
-#define BUILD(func, ...)                                                    \
-  (build(decoder) ? CheckForException(decoder, builder_->func(__VA_ARGS__)) \
-                  : nullptr)
+#define BUILD(func, ...)                                            \
+  ([&] {                                                            \
+    DCHECK(ssa_env_->go());                                         \
+    DCHECK(decoder->ok());                                          \
+    return CheckForException(decoder, builder_->func(__VA_ARGS__)); \
+  })()
 
 constexpr uint32_t kNullCatch = static_cast<uint32_t>(-1);
 
@@ -193,7 +193,7 @@ class WasmGraphBuildingInterface {
   void If(Decoder* decoder, const Value& cond, Control* if_block) {
     TFNode* if_true = nullptr;
     TFNode* if_false = nullptr;
-    BUILD(BranchNoHint, cond.node, &if_true, &if_false);
+    if (ssa_env_->go()) BUILD(BranchNoHint, cond.node, &if_true, &if_false);
     SsaEnv* end_env = ssa_env_;
     SsaEnv* false_env = Split(decoder, ssa_env_);
     false_env->control = if_false;
@@ -511,8 +511,6 @@ class WasmGraphBuildingInterface {
   TFBuilder* builder_;
   uint32_t current_catch_ = kNullCatch;
 
-  bool build(Decoder* decoder) { return ssa_env_->go() && decoder->ok(); }
-
   TryInfo* current_try_info(Decoder* decoder) {
     return decoder->control_at(decoder->control_depth() - 1 - current_catch_)
         ->try_info;
@@ -852,7 +850,6 @@ class WasmGraphBuildingInterface {
   void DoCall(WasmFullDecoder<validate, WasmGraphBuildingInterface>* decoder,
               TFNode* index_node, const Operand& operand, const Value args[],
               Value returns[], bool is_indirect) {
-    if (!build(decoder)) return;
     int param_count = static_cast<int>(operand.sig->parameter_count());
     TFNode** arg_nodes = builder_->Buffer(param_count + 1);
     TFNode** return_nodes = nullptr;