[BOLT] Use direct storage for Label annotations. NFCI. (#70147)

Store the Label annotation directly in the operand and avoid the extra
allocation and indirection overheads associated with MCSimpleAnnotation.

bolt/include/bolt/Core/MCPlusBuilder.h

@@ -1181,7 +1181,7 @@ class MCPlusBuilder {
 
   /// Set the label of \p Inst. This label will be emitted right before \p Inst
   /// is emitted to MCStreamer.
-  bool setLabel(MCInst &Inst, MCSymbol *Label, AllocatorIdTy AllocatorId = 0);
+  bool setLabel(MCInst &Inst, MCSymbol *Label) const;
 
   /// Return MCSymbol that represents a target of this instruction at a given
   /// operand number \p OpNum. If there's no symbol associated with
@@ -1816,6 +1816,8 @@ class MCPlusBuilder {
   const ValueType &addAnnotation(MCInst &Inst, unsigned Index,
                                  const ValueType &Val,
                                  AllocatorIdTy AllocatorId = 0) {
+    assert(Index >= MCPlus::MCAnnotation::kGeneric &&
+           "Generic annotation type expected.");
     assert(!hasAnnotation(Inst, Index));
     AnnotationAllocator &Allocator = getAnnotationAllocator(AllocatorId);
     auto *A = new (Allocator.ValueAllocator)

bolt/lib/Core/MCPlusBuilder.cpp

@@ -267,15 +267,15 @@ bool MCPlusBuilder::clearOffset(MCInst &Inst) const {
 }
 
 MCSymbol *MCPlusBuilder::getLabel(const MCInst &Inst) const {
-  if (auto Label = tryGetAnnotationAs<MCSymbol *>(Inst, MCAnnotation::kLabel))
-    return *Label;
+  if (std::optional<int64_t> Label =
+          getAnnotationOpValue(Inst, MCAnnotation::kLabel))
+    return reinterpret_cast<MCSymbol *>(*Label);
   return nullptr;
 }
 
-bool MCPlusBuilder::setLabel(MCInst &Inst, MCSymbol *Label,
-                             AllocatorIdTy AllocatorId) {
-  getOrCreateAnnotationAs<MCSymbol *>(Inst, MCAnnotation::kLabel, AllocatorId) =
-      Label;
+bool MCPlusBuilder::setLabel(MCInst &Inst, MCSymbol *Label) const {
+  setAnnotationOpValue(Inst, MCAnnotation::kLabel,
+                       reinterpret_cast<int64_t>(Label));
   return true;
 }