[AArch64] Improve load/store optimizer to handle LDUR + LDR (take 3).

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 and r259790.  The tramp3d failure was caused
by an incorrect refactoring in the patch.  Specifically, we weren't always
properly clearing the SExtIdx flag.

llvm-svn: 259812