[SystemZ] Mark fminimumnum/fmaximumnum as legal (#184595)
In M=4 mode, the behavior matches IEEE 754-2019 minimumNumber, except
that if both operands are sNaN, the result will be sNaN rather than
qNaN. However, this is explicitly allowed for LLVM's minimumnum
intrinsic, as canonicalization can be omitted for non-constrainted FP.
As such, mark fminimumnum/fmaximumnum as legal, and lower them the same
way as fminnum/fmaxnum. In the future, we may wish to switch those to
use M=0 instead, to match IEEE 754-2008 maxNum/minNum instead.
