[AArch64] Optimize some simple TBZ/TBNZ cases.

Summary:
Add some AArch64 dag combines to optimize some simple TBZ/TBNZ cases:

 (tbz (and x, m), b) -> (tbz x, b)
 (tbz (shl x, c), b) -> (tbz x, b-c)
 (tbz (shr x, c), b) -> (tbz x, b+c)
 (tbz (xor x, -1), b) -> (tbnz x, b)

Reviewers: jmolloy, mcrosier, t.p.northover

Subscribers: aemerson, rengolin, llvm-commits

Differential Revision: http://reviews.llvm.org/D15702

llvm-svn: 256765