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

[Ryotaro Kasuga via llvm-commits]

================
@@ -898,6 +947,77 @@ static Value *followLCSSA(Value *SV) {
   return followLCSSA(PHI->getIncomingValue(0));
 }
 
+bool CheckReductionKind(Loop *L, PHINode *PHI,
----------------
kasuga-fj wrote:

```suggestion
static bool checkReductionKind(Loop *L, PHINode *PHI,
```
See [Coding Standards](https://llvm.org/docs/CodingStandards.html#name-types-functions-variables-and-enumerators-properly).

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