R600/SI: Fixup CopyToReg register class in PostprocessISelDAG()

The CopyToReg nodes will sometimes try to copy a value from a VGPR to an
SGPR.  This kind of copy is not possible, so we need to detect
VGPR->SGPR copies and do something else.  The current strategy is to
replace these copies with VGPR->VGPR copies and hope that all the users
of CopyToReg can accept VGPRs as arguments.

llvm-svn: 183132