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

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon requested changes to this pull request.

Its not acceptable to have target specific like this in the generic DAG combines - either you need to convert the NaNs in MIPS ISD::ConstantFP custom lowering or in MipsISelDAGToDAG or (if we have to......) consider a TLI.getNaN() callback

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