CodeGen: Surface shouldRewriteCopySrc utility function (#158524)

Change shouldRewriteCopySrc to return the common register
class and expose it as a utility function. I've found myself
reproducing essentially the same logic in multiple places. The
purpose of this function is to jsut work through the API constraints
of which combination of register class and subreg indexes you have.

i.e. you need to use a different function if you have 0, 1, or 2
subregister indexes involved in a pair of copy-like operations.

0 files changed, 0 insertions, 0 deletions