[llvm] [InstCombine] Support well-defined recurrences in isGuaranteedNotToBeUndefOrPoison (PR #150420)

[Cullen Rhodes via llvm-commits]

================
@@ -7657,6 +7717,13 @@ static bool isGuaranteedNotToBeUndefOrPoison(
       }
       if (IsWellDefined)
         return true;
+
+      bool StartNeedsFreeze;
+      if (canFoldFreezeIntoRecurrence(const_cast<PHINode *>(PN),
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c-rhodes wrote:

I ran ctmark and the difference is negligible, but I suppose it's not a panacea, so I can if we think it's necessary. Although I'm not sure what a reasonable number is? tbh the thing I care about it's only 2 incoming values so I could restrict it to that.

https://github.com/llvm/llvm-project/pull/150420