tcg/optimize: Remember garbage high bits for 32-bit ops

For a 64-bit host, the high bits of a register after a 32-bit operation
are undefined.  Adjust the temps mask for all 32-bit ops to reflect that.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Richard Henderson <rth@twiddle.net>