PPC: Add ConstantPoolBuilder dtor to fix DCHECK error

When a failing compilation occur, constant pool may not be emitted but
linked (at the starting of code sequence). When CodeGenerator
destructor is called, Assembler, ConstantPoolBuilder destructor are
also called, which lead to calling the destructor of the emitted_label_,
and then DCHECK fails because it's linked.

Bug: v8:8785
Change-Id: I8926baa2fd7c16c462eef0c146d92ac7bf355396
Reviewed-on: https://chromium-review.googlesource.com/c/1452350Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#59371}

src/constant-pool.h

@@ -88,6 +88,14 @@ class ConstantPoolBuilder {
  public:
   ConstantPoolBuilder(int ptr_reach_bits, int double_reach_bits);
 
+#ifdef DEBUG
+  ~ConstantPoolBuilder() {
+    // Unused labels to prevent DCHECK failures.
+    emitted_label_.Unuse();
+    emitted_label_.UnuseNear();
+  }
+#endif
+
   // Add pointer-sized constant to the embedded constant pool
   ConstantPoolEntry::Access AddEntry(int position, intptr_t value,
                                      bool sharing_ok) {