target-ppc: Use 32-bit rotate instead of deposit + 64-bit rotate

A 32-bit rotate insn is more common on hosts than a deposit insn,
and if the host has neither the result is truely horrific.

At the same time, tidy up the temporaries within these functions,
drop the over-use of "likely", drop some checks for identity that
will also be checked by tcg-op.c functions, and special case mask
without rotate within rlwinm.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
