Recommit r218010 [FastISel][AArch64] Fold bit test and branch into TBZ and TBNZ.

Note: This version fixed an issue with the TBZ/TBNZ instructions that were
generated in FastISel. The issue was that the 64bit version of TBZ (TBZX)
automagically sets the upper bit of the immediate field that is used to specify
the bit we want to test. To test for any of the lower 32bits we have to first
extract the subregister and use the 32bit version of the TBZ instruction (TBZW).

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