[AArch64] Add UABDL patterns for log2 shuffle.

Summary:
This matches the sum-of-absdiff patterns emitted by the vectoriser using log2 shuffles.

Relies on D14207 to be able to match the `extract_subvector(..., 0)`

Reviewers: t.p.northover, jmolloy

Subscribers: aemerson, llvm-commits, rengolin

Differential Revision: http://reviews.llvm.org/D14208

llvm-svn: 252465