[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>

llvm-svn: 231527