ART: Special-case cb(n)z even for in-reg constant

Call out to OpCmpImmBranch in GenCompareAndBranch if the constant
is zero and we are testing == or !=, even when zero has been loaded
to a register already. This avoids a register size mismatch on 64b
architectures when basically doing a null check, and generally
squashes a cmp + branch to a cbz or cbnz on Arm and Mips. X86 is
not degraded.

Bug: 16562601
Change-Id: I1997760f43dc186a84247ad30ae91053f71d102d