JIT: Allow use of cbz/cbnz for conditional branch gen.

Optimize conditional branch generation when comparing
with zero for ARM JIT. When possible use cbz/cbnz
instead of cmp+beq/cmp+bne.

This improves performance for usecases involving
code execution in Dalvik.

Change-Id: I8f7c44c87eb73d4da00a48fd86220a62cb0515f5
Signed-off-by: Patrik Ryd <patrik.ryd@stericsson.com>