AMDGPU/GlobalISel: Manually RegBankSelect copies

This was failng on any pre-assigned copy to the VCC bank.

This is something of a workaround for the default implementation in
getInstrMappingImpl, and how it treats copy-like operations in
general.

Copy-like operations are considered to only have one result register
bank, rather than separate banks for each source like a normal
instruction. To avoid potentially mishandling reg_sequence with
impossible operand combinations, the generic implementation errors on
impossible costs. If the bank was already assigned, is treated it
as-if it were an unsatisfiable REG_SEQUENCE mapping. We really don't
get any value from any of what getInstrMappingImpl tries to do for
copies, so just directly emit the simple mapping we really want.