CodeGenPrep: wrangle IR to exploit AArch64 tbz/tbnz inst.

Given IR like:
    %bit = and %val, #imm-with-1-bit-set
    %tst = icmp %bit, 0
    br i1 %tst, label %true, label %false

some targets can emit just a single instruction (tbz/tbnz in the
AArch64 case). However, with ISel acting at the basic-block level, all
three instructions need to be together for this to be possible.

This adds another transformation to CodeGenPrep to expose these
opportunities, if targets opt in via the hook.

llvm-svn: 205086