[AArch64][GlobalISel] Optimize conditional branches followed by unconditional branches

If we have an icmp->brcond->br sequence where the brcond just branches to the
next block jumping over the br, while the br takes the false edge, then we can
modify the conditional branch to jump to the br's target while inverting the
condition of the incoming icmp. This means we can eliminate the br as an
unconditional branch to the fallthrough block.

Differential Revision: https://reviews.llvm.org/D64354

llvm-svn: 365510