[llvm] [IR][DAG] Add support for `nneg` flag with `uitofp` (PR #86141)

[Matt Arsenault via llvm-commits]

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@@ -3005,6 +3005,12 @@ class TargetLoweringBase {
     return false;
   }
 
+  /// Return true if sitofp from FromTy to ToTy is cheaper than
+  /// uitofp.
+  virtual bool isSIToFPCheaperThanUIToFP(EVT FromTy, EVT ToTy) const {
+    return false;
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arsenm wrote:

I really dislike adding random target hooks like this, especially without reasonable defaults. Just check which of the two is legal/custom like most places? 

https://github.com/llvm/llvm-project/pull/86141