[AArch64][GlobalISel] Implement custom legalization for s32/s64 G_FCOPYSIGN

This is intended to be equivalent to the s32 + s64 cases in
AArch64TargetLowering::LowerFCOPYSIGN.

Widen everything and then use G_BIT + a mask to handle the actual copysign
operation. Then, narrow back down to s32/s64.

I wasn't sure about what the best/most canonical INSERT_SUBREG-selectable
pattern is. I chose G_INSERT_VECTOR_ELT + an undef vector because it produces
reasonably okay codegen. (It doesn't produce INSERT_SUBREG right now though.)
If there's a better way to do this then I'm happy to change it.

We also have a couple codegen deficiencies with how we emit vector constants
right now. (We need a GISel equivalent to the tryAdvSIMDModImm64 stuff)

Differential Revision: https://reviews.llvm.org/D108725