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

[Matt Arsenault via llvm-commits]

arsenm wrote:

LLVM as a whole does not support the legacy mips nan encoding. At best you're doing spot fixes for specific issues someone encounters. We should probably just turn the flag into an error; it would be a huge undertaking to actually support it correctly 

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