[RegBankSelect] Explain what it would take to support non-copy

repairing.

Copies are easy because we repair only when there is a mismatch. For
non-copy repairing, i.e., cases that involves breaking down or gathering
up the value, one of the operand may not have a register bank yet. Thus,
derivate a cost from that, requires more work.

llvm-svn: 272157