[AArch64] Genereate CCMP from And CSel

LLVM has a couple of ways of producing ccmp - either from chains in isel
or from a later ifcvt style pass. This adds a simple DAG combine to
capture more cases, converting and(csel(0, 1, cc0), csel(0, 1, cc1))
into a csel(ccmp(.., cc0)), depending on cc1 (a SUBS in this case).

Differential Revision: https://reviews.llvm.org/D118327