[AArch64][GlobalISel] Use fcsel instead of csel for G_SELECT on FPRs

This saves us some unnecessary copies.

If the inputs to a G_SELECT are floating point, we should use fcsel rather than
csel.

Changes here are...

- Teach selectCopy about s1-to-s1 copies across register banks.
- AArch64RegisterBankInfo about G_SELECT in general.
- Teach the instruction selector about the FCSEL instructions.

Also add two tests:

- select-select.mir to show that we get the expected FCSEL
- regbank-select.mir (unfortunately named) to show the register banks on
G_SELECT are properly preserved

And update fast-isel-select.ll to show that we do the same thing as other
instruction selectors in these cases.

llvm-svn: 359940