[AArch64] Improve load/store optimizer to handle LDUR + LDR.

This patch allows the mixing of scaled and unscaled load/stores to form
load/store pairs.

PR24465
http://reviews.llvm.org/D12116
Many thanks to Ahmed and Michael for fixes and code review.

This is a reapplication of r246769, which was reverted in r246782 due to a
test-suite failure.  I'm unable to reproduce the issue at this time.

llvm-svn: 259790