[llvm] [KnownFPClass] Add neg_square and fma_neg_square (PR #227248)

[Matt Arsenault via llvm-commits]

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@@ -287,6 +287,27 @@ struct KnownFPClass {
     return Known;
   }
 
+  // Special cases of fmul -x, x and fmul x, -x.
+  static KnownFPClass
+  neg_square(const KnownFPClass &Src,
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arsenm wrote:

Can move this out of line 

https://github.com/llvm/llvm-project/pull/227248