[AArch64] Fix incorrectly getting the destination reg of an insn (#101205)

This popped up while investigating
#96950
In a few places where we need the destination reg of an instruction we
were using a call that worked only by accident.

Author: momchil-velikov

llvm/lib/Target/AArch64/AArch64InstrInfo.cpp

@@ -4112,6 +4112,7 @@ bool AArch64InstrInfo::isPairedLdSt(const MachineInstr &MI) {
 }
 
 const MachineOperand &AArch64InstrInfo::getLdStBaseOp(const MachineInstr &MI) {
+  assert(MI.mayLoadOrStore() && "Load or store instruction expected");
   unsigned Idx =
       AArch64InstrInfo::isPairedLdSt(MI) || AArch64InstrInfo::isPreLdSt(MI) ? 2
                                                                             : 1;
@@ -4120,6 +4121,7 @@ const MachineOperand &AArch64InstrInfo::getLdStBaseOp(const MachineInstr &MI) {
 
 const MachineOperand &
 AArch64InstrInfo::getLdStOffsetOp(const MachineInstr &MI) {
+  assert(MI.mayLoadOrStore() && "Load or store instruction expected");
   unsigned Idx =
       AArch64InstrInfo::isPairedLdSt(MI) || AArch64InstrInfo::isPreLdSt(MI) ? 3
                                                                             : 2;

llvm/lib/Target/AArch64/AArch64LoadStoreOptimizer.cpp

@@ -1951,12 +1951,15 @@ AArch64LoadStoreOpt::findMatchingInsn(MachineBasicBlock::iterator I,
 
 static MachineBasicBlock::iterator
 maybeMoveCFI(MachineInstr &MI, MachineBasicBlock::iterator MaybeCFI) {
+  assert((MI.getOpcode() == AArch64::SUBXri ||
+          MI.getOpcode() == AArch64::ADDXri) &&
+         "Expected a register update instruction");
   auto End = MI.getParent()->end();
   if (MaybeCFI == End ||
       MaybeCFI->getOpcode() != TargetOpcode::CFI_INSTRUCTION ||
       !(MI.getFlag(MachineInstr::FrameSetup) ||
         MI.getFlag(MachineInstr::FrameDestroy)) ||
-      AArch64InstrInfo::getLdStBaseOp(MI).getReg() != AArch64::SP)
+      MI.getOperand(0).getReg() != AArch64::SP)
     return End;
 
   const MachineFunction &MF = *MI.getParent()->getParent();
@@ -2006,7 +2009,7 @@ AArch64LoadStoreOpt::mergeUpdateInsn(MachineBasicBlock::iterator I,
   if (!AArch64InstrInfo::isPairedLdSt(*I)) {
     // Non-paired instruction.
     MIB = BuildMI(*I->getParent(), I, I->getDebugLoc(), TII->get(NewOpc))
-              .add(getLdStRegOp(*Update))
+              .add(Update->getOperand(0))
               .add(getLdStRegOp(*I))
               .add(AArch64InstrInfo::getLdStBaseOp(*I))
               .addImm(Value / Scale)
@@ -2015,7 +2018,7 @@ AArch64LoadStoreOpt::mergeUpdateInsn(MachineBasicBlock::iterator I,
   } else {
     // Paired instruction.
     MIB = BuildMI(*I->getParent(), I, I->getDebugLoc(), TII->get(NewOpc))
-              .add(getLdStRegOp(*Update))
+              .add(Update->getOperand(0))
               .add(getLdStRegOp(*I, 0))
               .add(getLdStRegOp(*I, 1))
               .add(AArch64InstrInfo::getLdStBaseOp(*I))