[MIPS] Use softPromoteHalf legalization for fp16 rather than PromoteFloat

Fix part of #97975.

Author: yingopq

llvm/lib/Target/Mips/MipsISelLowering.h

@@ -365,6 +365,8 @@ class TargetRegisterClass;
       return ABI.IsN64() ? Mips::A1_64 : Mips::A1;
     }
 
+    bool softPromoteHalfType() const override { return true; }
+
     bool isJumpTableRelative() const override {
       return getTargetMachine().isPositionIndependent();
     }