Apply the no-r0 register class to the PPC SELECT_CC_I[4|8] pseudos

Either operand of these pseudo instructions can be transformed into the first
operand of an isel instruction (and this operand cannot be r0).

This is not yet testable because we don't yet allocate r0 (and even when we do,
any test would be very fragile).

llvm-svn: 178119