R600/SI: Move SIFixSGPRCopies to inst selector passes

This should expose more of the actually used VALU
instructions to the machine optimization passes.

This also should help getting i1 handling into a better state.
For not entirly understood reasons, this fixes the split-scalar-i64-add.ll
test where a 64-bit add would only partially be moved to the VALU
resulting in use of undefined VCC.