[AArch64] Check Dest Register Liveness in CondOpt pass.

Our internal test reveals such case should not be transformed:

  cmp x17, #3
  b.lt .LBB10_15
  ...
  subs x12, x12, #1
  b.gt .LBB10_1

where x12 is a liveout, becomes:

  cmp x17, #2
  b.le .LBB10_15
  ...
  subs x12, x12, #2
  b.ge .LBB10_1

Unable to provide test case as it's difficult to reproduce on community branch.

http://reviews.llvm.org/D6048
Patch by Zhaoshi Zheng <zhaoshiz@codeaurora.org>!

llvm-svn: 220987