[AArch64][GlobalISel] NFC: Refactor G_FCMP selection code

Refactor this so it's similar to the existing integer comparison code.

Also add some missing 64-bit testcases to select-fcmp.mir.

Refactoring to prep for improving selection for G_FCMP-related conditional
branches etc.

Differential Revision: https://reviews.llvm.org/D88614