[AArch64] Turn on by default interleaved access vectorization

Summary:
This change turns on by default interleaved access vectorization
for AArch64.

We also clean up some tests which were spedifically enabling this
behaviour.

Reviewers: rengolin

Subscribers: aemerson, llvm-commits, rengolin

Differential Revision: http://reviews.llvm.org/D12149

llvm-svn: 246542