[AArch64][LoadStoreOptimizer] Generate LDP + SXTW instead of LD[U]R + LD[U]RSW.

[AArch64][LoadStoreOptimizer] Generate LDP + SXTW instead of LD[U]R + LD[U]RSW.
Teach the load store optimizer how to sign extend a result of a load pair when
it helps creating more pairs.
The rational is that loads are more expensive than sign extensions, so if we
gather some in one instruction this is better!

<rdar://problem/20072968>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231527 91177308-0d34-0410-b5e6-96231b3b80d8