[wasm] Skip end env for outermost block

Since implicit returns are gone, this environment is never being used.

R=titzer@chromium.org

Bug: v8:8423
Change-Id: I24d61a1316433fde6835fd608c3d308592721a5c
Reviewed-on: https://chromium-review.googlesource.com/c/1371569
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58155}

src/wasm/graph-builder-interface.cc

@@ -131,11 +131,7 @@ class WasmGraphBuildingInterface {
     builder_->InitInstanceCache(&ssa_env->instance_cache);
   }
 
-  void StartFunctionBody(FullDecoder* decoder, Control* block) {
-    SsaEnv* branch_env = ssa_env_;
-    SetEnv(Steal(decoder->zone(), branch_env));
-    block->end_env = branch_env;
-  }
+  void StartFunctionBody(FullDecoder* decoder, Control* block) {}
 
   void FinishFunction(FullDecoder*) { builder_->PatchInStackCheckIfNeeded(); }