[AArch64] Keep track of MIFlags in the LoadStoreOptimizer

Merging:

* $x26, $x25 = frame-setup LDPXi $sp, 0
* $sp = frame-destroy ADDXri $sp, 64, 0

into an LDPXpost should preserve the flags from both instructions as
following:

* frame-setup frame-destroy LDPXpost

Differential Revision: https://reviews.llvm.org/D44446

llvm-svn: 327533