[FastISel][AArch64] Fold bit test and branch into TBZ and TBNZ.

Teach selectBranch to fold bit test and branch into a single instruction (TBZ or
TBNZ).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218010 91177308-0d34-0410-b5e6-96231b3b80d8