[llvm] [LoopInterchange] Support inner-loop simple reductions via UndoSimpleReduction (PR #172970)

[Michael Kruse via llvm-commits]

https://github.com/Meinersbur commented:

Could we chose a different name than "UndoSimpleReduction"? The opposite of mem2reg is not "UndoMem2Reg", but reg2mem. The reduction is not necessarily undoing a register promotion, but could emerge from a series of optimizations. In that case "undo" would be misleading.

Suggestions:
 * DemoteReduction (see `llvm::DemoteRegToStack`)
 * Reduction2Mem (see `reg2mem` pass)


https://github.com/llvm/llvm-project/pull/172970