R600/SI: Fix live range error hidden by SIFoldOperands

R600/SI: Fix live range error hidden by SIFoldOperands

m0 is treated as a virtual register class with a single register
rather than the physical register it really is. This was updating
the live range of the used virtual copy of m0 from the first ds_read
instruction, and leaving the unused copy unchanged. This resulted in a
"Live segment doesn't end at a valid instruction" verifier error because
the erased instructions. Update the live range of the second copy (which
should be dead).

No test since I'm not sure how to trigger this with SIFoldOperands
enabled.

llvm-svn: 223203