[AArch64][GlobalISel] Avoid copies to target register bank for subregister copies

Previously for any copy from a register bigger than the destination:

Copied to a same-sized register in the destination register bank.
Subregister copy of that to the destination.
This fails for copies from 128-bit FPRs to GPRs because the GPR register bank
can't accomodate 128-bit values.

Instead of special-casing such copies to perform the truncation beforehand in
the source register bank, generalize this:
a) Perform a subregister copy straight from source register whenever possible.
This results in shorter MIR and fixes the above problem.

b) Perform a full copy to target bank and then do a subregister copy only if
source bank can't support target's size. E.g. GPR to 8-bit FPR copy.

Patch by Raul Tambre (tambre)!

Differential Revision: https://reviews.llvm.org/D75421