[llvm] [InstCombine] Refactor fixed and scalable binop shuffle combine. NFCI (PR #141287)

[Yingwei Zheng via llvm-commits]

================
@@ -2094,6 +2094,50 @@ static bool shouldMergeGEPs(GEPOperator &GEP, GEPOperator &Src) {
   return true;
 }
 
+// Find constant NewC that has property:
----------------
dtcxzyw wrote:

Use `///` for header comments.


https://github.com/llvm/llvm-project/pull/141287