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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@272157 91177308-0d34-0410-b5e6-96231b3b80d8

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