[AArch64] Stop creating unnecessary label MCSymbols for each Windows unwind opcode. NFC.

[mstorsjo]

Summary

These labels aren't needed in the ARM version of WinEH tables, as each
unwind opcode maps to a specific instruction (each opcode is assumed
to represent one instruction), and the written tables don't contain
offsets like on x86_64.

[rnk]

lgtm

llvm/lib/Target/AArch64/MCTargetDesc/AArch64WinCOFFStreamer.cpp

 // 2) take an offset: SEH_StackAlloc, SEH_SaveFPLR, SEH_SaveFPLR_X
 // 3) take a register and an offset/size: all others
 void AArch64TargetWinCOFFStreamer::emitARM64WinUnwindCode(unsigned UnwindCode,
                                                           int Reg, int Offset) {
   auto &S = getStreamer();
   WinEH::FrameInfo *CurFrame = S.EnsureValidWinFrameInfo(SMLoc());
   if (!CurFrame)
     return;
-  MCSymbol *Label = S.emitCFILabel();
-  auto Inst = WinEH::Instruction(UnwindCode, Label, Reg, Offset);
+  auto Inst = WinEH::Instruction(UnwindCode, /*Label=*/nullptr, Reg, Offset);
   if (InEpilogCFI)
     CurFrame->EpilogMap[CurrentEpilog].push_back(Inst);
   else
     CurFrame->Instructions.push_back(Inst);
 }
 
 void AArch64TargetWinCOFFStreamer::emitARM64WinCFIAllocStack(unsigned Size) {
   unsigned Op = Win64EH::UOP_AllocSmall;

 void AArch64TargetWinCOFFStreamer::emitARM64WinCFIPrologEnd() {
   auto &S = getStreamer();
   WinEH::FrameInfo *CurFrame = S.EnsureValidWinFrameInfo(SMLoc());
   if (!CurFrame)
     return;
 
   MCSymbol *Label = S.emitCFILabel();
   CurFrame->PrologEnd = Label;
-  WinEH::Instruction Inst = WinEH::Instruction(Win64EH::UOP_End, Label, -1, 0);
+  WinEH::Instruction Inst =
+      WinEH::Instruction(Win64EH::UOP_End, /*Label=*/nullptr, -1, 0);
   auto it = CurFrame->Instructions.begin();
   CurFrame->Instructions.insert(it, Inst);
 }
 
 void AArch64TargetWinCOFFStreamer::emitARM64WinCFIEpilogStart() {
   auto &S = getStreamer();
   WinEH::FrameInfo *CurFrame = S.EnsureValidWinFrameInfo(SMLoc());
   if (!CurFrame)
     return;
 
   InEpilogCFI = true;
   CurrentEpilog = S.emitCFILabel();
 }
 
 void AArch64TargetWinCOFFStreamer::emitARM64WinCFIEpilogEnd() {
   auto &S = getStreamer();
   WinEH::FrameInfo *CurFrame = S.EnsureValidWinFrameInfo(SMLoc());
   if (!CurFrame)
     return;
 
   InEpilogCFI = false;
-  MCSymbol *Label = S.emitCFILabel();
-  WinEH::Instruction Inst = WinEH::Instruction(Win64EH::UOP_End, Label, -1, 0);
+  WinEH::Instruction Inst =
+      WinEH::Instruction(Win64EH::UOP_End, /*Label=*/nullptr, -1, 0);
   CurFrame->EpilogMap[CurrentEpilog].push_back(Inst);
   CurrentEpilog = nullptr;
 }
 
 void AArch64TargetWinCOFFStreamer::emitARM64WinCFITrapFrame() {
   emitARM64WinUnwindCode(Win64EH::UOP_TrapFrame, -1, 0);
 }