MIPSr6: Set FMAXNUM and FMINNUM as Legal (#139009)
Now we define FMAXNUM and FMINNUM as IEEE754-2008 with +0.0>-0.0.
MIPSr6's fmax/fmin just follow this rules full.
FMAXNUM_IEEE and FMINNUM_IEEE will be removed in future once:
1. Fixes FMAXNUM/FMINNUM for all targets
2. The use of FMAXNUM_IEEE/FMINNUM_IEEE are not used by middle end
anymore.
