[AArch64][GlobalISel] Import FMOV patterns rather than manually selecting it

There are existing patterns for FMOVHi, FMOVSi, and FMOVDi in
AArch64InstrFormats.td.

Importing these allows us to remove the manual selection code for FMOV.

It also allows us to select FMOVHi for non-zero constants when we have full
fp-16 support.

Refactor some of the code in AArch64InstrFormats.td so that we can create
equivalent custom renderers in GlobalISel.

Differential Revision: https://reviews.llvm.org/D97511