[llvm] [MachineLateInstrsCleanup] Reuse redundant spill-slot reloads (PR #220180)

[via llvm-commits]

anupkum-sgs wrote:

Greedy already handles multiple stages:  assign-->split-->spill-->last chance recoloring. So I was hesitant to make changes in such a critical pass, as they could potentially interfere with decisions that might otherwise result in better register allocation. On other hand, MachineLateInstrsCleanup pass is already removing the removing redundant defs, so handling these redundant reloads feels like a small and natural extension of its existing functionality.

https://github.com/llvm/llvm-project/pull/220180