[llvm] [RegAllocBase] Produce IMPLICIT_DEF instead of COPY undef during cleanupFailedVReg (PR #147392)

[Matt Arsenault via llvm-commits]

arsenm wrote:

The problem isn't the undef copy, it's the fact that storeRegToStack slot doesn't propagate the undef flag from the source when folding the copy. Disabling the optimization works, but we alternatively could pass through the undef flag and let the target produce the undef spill. Or, not emit the spill in the first place (that is complicated since all this code expects a single instruction be produced. I guess we could instead insert an undef KILL in place of a store)

https://github.com/llvm/llvm-project/pull/147392