[AArch64] Generate tbz/tbnz when comparing against zero.

The tbz/tbnz checks the sign bit to convert

op w1, w1, w10
cmp w1, #0
b.lt .LBB0_0

to

op w1, w1, w10
tbnz w1, #31, .LBB0_0

Differential Revision: http://reviews.llvm.org/D4440