[wasm] Implement the unreachable-instruction with TrapUnless

With this change we can remove all the old trap code.

R=titzer@chromium.org, clemensh@chromium.org

Change-Id: I85c10a6ac7d3eccc7b611e06f2a651d5a8a00a9c
Reviewed-on: https://chromium-review.googlesource.com/452379Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43853}

src/compiler/wasm-compiler.cc

@@ -1147,10 +1147,12 @@ Node* WasmGraphBuilder::Return(unsigned count, Node** vals) {
   return ret;
 }
 
-Node* WasmGraphBuilder::ReturnVoid() { return Return(0, Buffer(0)); }
+Node* WasmGraphBuilder::ReturnVoid() { return Return(0, nullptr); }
 
 Node* WasmGraphBuilder::Unreachable(wasm::WasmCodePosition position) {
-  trap_->Unreachable(position);
+  trap_->AddTrapIfFalse(wasm::TrapReason::kTrapUnreachable, Int32Constant(0),
+                        position);
+  ReturnVoid();
   return nullptr;
 }