[AArch64][GlobalISel] Mark G_FMINNUM/G_FMAXNUM as floating point opcodes

We need to ensure that these end up on FPR to allow imported patterns to
select them.

This will also ensure that we get good regbank selection when dealing with
instructions like G_PHI/G_LOAD/G_STORE which deduce their banks from their
uses/users.

Differential Revision: https://reviews.llvm.org/D108260