[csa] Fix the bound & used condition check for RawMachineLabel and CodeAssemblerLabel.

CodeAssemblerLabel's destructor was not calling RawMachineLabel's destructor, because label_ is zone-allocated. RawMachineLabel's destructor contains a DCHECK that would make debugging easier. If the DCHECK is not triggered, things will go awry in the register allocation phase, making it harder to debug the issue.

BUG=

Review-Url: https://codereview.chromium.org/2641863002
Cr-Commit-Position: refs/heads/master@{#42461}

src/compiler/code-assembler.cc

@@ -719,6 +719,8 @@ CodeAssemblerLabel::CodeAssemblerLabel(CodeAssembler* assembler,
   }
 }
 
+CodeAssemblerLabel::~CodeAssemblerLabel() { label_->~RawMachineLabel(); }
+
 void CodeAssemblerLabel::MergeVariables() {
   ++merge_count_;
   for (auto var : state_->variables_) {

src/compiler/code-assembler.h

@@ -444,7 +444,7 @@ class CodeAssemblerLabel {
       CodeAssembler* assembler, CodeAssemblerVariable* merged_variable,
       CodeAssemblerLabel::Type type = CodeAssemblerLabel::kNonDeferred)
       : CodeAssemblerLabel(assembler, 1, &merged_variable, type) {}
-  ~CodeAssemblerLabel() {}
+  ~CodeAssemblerLabel();
 
  private:
   friend class CodeAssembler;

src/compiler/raw-machine-assembler.cc

@@ -332,7 +332,11 @@ Node* RawMachineAssembler::MakeNode(const Operator* op, int input_count,
   return graph()->NewNodeUnchecked(op, input_count, inputs);
 }
 
-RawMachineLabel::~RawMachineLabel() { DCHECK(bound_ || !used_); }
+RawMachineLabel::~RawMachineLabel() {
+  // If this DCHECK fails, it means that the label has been bound but it's not
+  // used, or the opposite. This would cause the register allocator to crash.
+  DCHECK_EQ(bound_, used_);
+}
 
 }  // namespace compiler
 }  // namespace internal