[AArch64][GlobalISel] Add post-legalizer lowering for NEON vector fcmps

This is roughly equivalent to the floating point portion of
`AArch64TargetLowering::LowerVSETCC`. Main part that's missing is the v4s16 bit.

This also adds helpers equivalent to `EmitVectorComparison`, and
`changeVectorFPCCToAArch64CC`. This moves `changeFCMPPredToAArch64CC` out of
the selector into AArch64GlobalISelUtils for the sake of code reuse.

This is done in post-legalizer lowering with pseudos to simplify selection.
The imported patterns end up handling selection for us this way.

Differential Revision: https://reviews.llvm.org/D101782