The HW does not special-case r0, but the ABI specifies that r0 should
contain 0. If we expose this fact to the optimizer, we can simplify
a lot of the generated code. We must of course verify that r0==0, but
that is trivial to do with a TB flag.
Signed-off-by: Richard Henderson <rth@twiddle.net>
6597c28d61