AMDGPU/GlobalISel: Calculate isKnownNeverNaN for fminnum and fmaxnum

Implements same logis as in SelectionDAG.
G_FMINNUM_IEEE and G_FMAXNUM_IEEE are never SNaN by definition and
never NaN when one operand is known non-NaN and other known non-SNaN.
G_FMINNUM and G_FMAXNUM are never NaN/SNaN when one of the operands
is known non-NaN/SNaN.

Differential Revision: https://reviews.llvm.org/D91716