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

[Ramkumar Ramachandra via llvm-commits]

================
@@ -736,11 +757,18 @@ RecurrenceDescriptor::isFindLastIVPattern(Loop *TheLoop, PHINode *OrigPhi,
   // We are looking for selects of the form:
   //   select(cmp(), phi, increasing_loop_induction) or
   //   select(cmp(), increasing_loop_induction, phi)
-  // TODO: Support for monotonically decreasing induction variable
-  if (!IsIncreasingLoopInduction(NonRdxPhi))
+  if (Kind == RecurKind::FindLastIV) {
+    if (IsSupportedLoopInduction(NonRdxPhi, Kind))
+      return InstDesc(I, Kind);
     return InstDesc(false, I);
+  }
+
+  if (IsSupportedLoopInduction(NonRdxPhi, RecurKind::FindFirstIVSMin))
+    return InstDesc(I, RecurKind::FindFirstIVSMin);
+  if (IsSupportedLoopInduction(NonRdxPhi, RecurKind::FindFirstIVUMin))
+    return InstDesc(I, RecurKind::FindFirstIVUMin);
 
-  return InstDesc(I, RecurKind::FindLastIV);
+  return InstDesc(false, I);
----------------
artagnon wrote:

Get IsSupportedLoopInduction to return a Kind, and compare with `Kind ==` to unify this code?

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