AArch64LoadStoreOptimizer: Update kill flags when merging stores

Kill flags need to be updated correctly when moving stores up/down to
form store pair instructions.
Those invalid flags have been ignored before but as of r290014 they are
recognized when using -mllvm -verify-machineinstrs.

Also simplifies test/CodeGen/AArch64/ldst-opt-dbg-limit.mir, renames it
to ldst-opt.mir test and adds a new tests for this change.

Differential Revision: https://reviews.llvm.org/D28875

llvm-svn: 292625