[wasm] Handle break to outermost block like a return

This saves a few control merges in Liftoff, and might also generate
smaller graphs in Turbofan.

R=titzer@chromium.org

Bug: v8:6600, v8:8423
Change-Id: Ice921f8b048809bc38b820b94688f482e67bd386
Reviewed-on: https://chromium-review.googlesource.com/c/1354039Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57907}

src/wasm/function-body-decoder-impl.h

@@ -1822,10 +1822,14 @@ class WasmFullDecoder : public WasmDecoder<validate> {
             BreakDepthImmediate<validate> imm(this, this->pc_);
             if (!this->Validate(this->pc_, imm, control_.size())) break;
             Control* c = control_at(imm.depth);
-            if (!TypeCheckBreak(c)) break;
-            if (control_.back().reachable()) {
-              CALL_INTERFACE(Br, c);
-              c->br_merge()->reached = true;
+            if (imm.depth == control_.size() - 1) {
+              DoReturn(c, false);
+            } else {
+              if (!TypeCheckBreak(c)) break;
+              if (control_.back().reachable()) {
+                CALL_INTERFACE(Br, c);
+                c->br_merge()->reached = true;
+              }
             }
             len = 1 + imm.length;
             EndControl();