[llvm] [RegAllocFast] Give an undef tied use the register of its tied def (PR #222249)

[Matt Arsenault via llvm-commits]

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@@ -1031,6 +1031,28 @@ void RegAllocFastImpl::allocVirtRegUndef(MachineOperand &MO) {
   if (!shouldAllocateRegister(VirtReg))
     return;
 
+  // The def is freed before the uses are allocated, so a tie is the only
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arsenm wrote:

That seems like a difficult condition to enforce on every pass 

side note: I think the bigger problem here is the way codegen treats undef flags/IMPLICIT_DEF is really poison, but we lower undef to it 

https://github.com/llvm/llvm-project/pull/222249