[AArch64][RegisterBankInfo] Use the statically computed mappings for COPY

We use to resort on the generic implementation to get the mappings for
COPYs. The generic implementation resorts on table lookup and
dynamically allocated objects to get the valid mappings.

Given we already know how to map G_BITCAST and have the static mappings
for them, use that code path for COPY as well. This is much more
efficient.

Improve the compile time of RegBankSelect by up to 20%.

Note: When we eventually generate all the mappings via TableGen, we
wouldn't have to do that dance to shave compile time. The intent of this
change was to make sure that moving to static structure really pays off.

NFC.

llvm-svn: 315781