Apply the no-r0 class to PPC TOC ADDI[S] pseudo instructions

Like the addi/addis instructions themselves, these pseudo instructions also
cannot have r0 as their register parameter (because it will be interpreted as
the value 0).

This is not yet testable because we don't yet allocate r0 (and even when we do,
any regression test would be very fragile because it would depend on the
register allocator heuristics).
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).