[MIPS] match llvm.{min,max}num with {min,max}.fmt for R6 (#89021)

- The behavior is similar to UCOMISD on x86, which is also used to
compare two fp values, specifically on handling of NaNs.
- Update related tests regarding this change.
- The further goal is to implement `llvm.minimum` and `llvm.maximum`
intrinsics for MIPS R6 and Pre-R6.

Part of https://github.com/llvm/llvm-project/issues/64207