[llvm] [Mips] Fix wrong qNaN encoding when -mnan=legacy (PR #153777)

[Simon Pilgrim via llvm-commits]

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@@ -4433,6 +4433,8 @@ class LLVM_ABI TargetLowering : public TargetLoweringBase {
   ///
   virtual SDValue PerformDAGCombine(SDNode *N, DAGCombinerInfo &DCI) const;
 
+  virtual APFloat getNaNValue(EVT VT) const;
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RKSimon wrote:

Missing a short dxoygen description 

https://github.com/llvm/llvm-project/pull/153777