[llvm] [AMDGPU] Make SIShrinkInstructions pass return valid changed state (PR #168833)

[Matt Arsenault via llvm-commits]

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@@ -580,6 +591,7 @@ bool SIShrinkInstructions::shrinkScalarLogicOp(MachineInstr &MI) const {
     if (Dest->getReg().isVirtual() && SrcReg->isReg()) {
       MRI->setRegAllocationHint(Dest->getReg(), 0, SrcReg->getReg());
       MRI->setRegAllocationHint(SrcReg->getReg(), 0, Dest->getReg());
+      MoveIterator = true;
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arsenm wrote:

I'd rather turn this into a return enum of statuses. It's really 2 levels of change - actually changed the instructions, and changed the function but not the actual content (only MRI register hint fields) 

https://github.com/llvm/llvm-project/pull/168833