[llvm] [LV] Add support for cmp reductions with decreasing IVs. (PR #140451)

[Mel Chen via llvm-commits]

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@@ -56,6 +56,13 @@ enum class RecurKind {
   FindLastIV, ///< FindLast reduction with select(cmp(),x,y) where one of
               ///< (x,y) is increasing loop induction, and both x and y are
               ///< integer type.
+  FindFirstIVUMin, /// FindFirst reduction with select(icmp(),x,y) where one of
+                   ///< (x,y) is a decreasing loop induction, and both x and y
+                   ///< are integer type.
+  FindFirstIVSMin /// FindFirst reduction with select(icmp(),x,y) where one of
+                  ///< (x,y) is a decreasing loop induction, and both x and y
+                  ///< are integer type.
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Mel-Chen wrote:

This patch introduces too many new idioms at once.
To keep the patch reviewable and focused, I suggest we pick one direction to extend first:
either support unsigned FindLastIV, or add signed FindFirstIV support.

https://github.com/llvm/llvm-project/pull/140451