[wasm] Simplify error detection at end of functions

The {last_end_found_} field is redundant. If it's false, there will be
a control struct left on the stack.
Thus remove that field, and simplify the error detection condition.

R=titzer@chromium.org

Bug: v8:8423
Change-Id: I16ac4597f229c5e3abd923f8eb504f93afb82eb4
Reviewed-on: https://chromium-review.googlesource.com/c/1373788Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58198}

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

@@ -1421,8 +1421,7 @@ class WasmFullDecoder : public WasmDecoder<validate> {
         local_type_vec_(zone),
         stack_(zone),
         control_(zone),
-        args_(zone),
-        last_end_found_(false) {
+        args_(zone) {
     this->local_types_ = &local_type_vec_;
   }
 
@@ -1449,24 +1448,16 @@ class WasmFullDecoder : public WasmDecoder<validate> {
     DecodeFunctionBody();
     if (!this->failed()) CALL_INTERFACE(FinishFunction);
 
-    if (this->failed()) return this->TraceFailed();
-
-    if (!control_.empty()) {
-      // Generate a better error message whether the unterminated control
-      // structure is the function body block or an innner structure.
-      if (control_.size() > 1) {
-        this->error(control_.back().pc, "unterminated control structure");
-      } else {
-        this->error("function body must end with \"end\" opcode");
-      }
-      return TraceFailed();
-    }
-
-    if (!last_end_found_) {
+    // Generate a better error message whether the unterminated control
+    // structure is the function body block or an innner structure.
+    if (control_.size() > 1) {
+      this->error(control_.back().pc, "unterminated control structure");
+    } else if (control_.size() == 1) {
       this->error("function body must end with \"end\" opcode");
-      return false;
     }
 
+    if (this->failed()) return this->TraceFailed();
+
     if (FLAG_trace_wasm_decode_time) {
       double ms = decode_timer.Elapsed().InMillisecondsF();
       PrintF("wasm-decode %s (%0.3f ms)\n\n",
@@ -1535,7 +1526,6 @@ class WasmFullDecoder : public WasmDecoder<validate> {
   ZoneVector<Value> stack_;               // stack of values.
   ZoneVector<Control> control_;           // stack of blocks, loops, and ifs.
   ZoneVector<Value> args_;                // parameters of current block or call
-  bool last_end_found_;
 
   bool CheckHasMemory() {
     if (!VALIDATE(this->module_->has_memory)) {
@@ -1805,7 +1795,6 @@ class WasmFullDecoder : public WasmDecoder<validate> {
                 this->error(this->pc_ + 1, "trailing code after function end");
                 break;
               }
-              last_end_found_ = true;
               // The result of the block is the return value.
               TRACE_PART("\n" TRACE_INST_FORMAT, startrel(this->pc_),
                          "(implicit) return");