[AArch64] Load/store opt: Don't count transient instructions towards search limits.

Summary:
This change also changes findMatchingInsn and
findMatchingUpdateInsnForward to take DBG_VALUE opcodes into account
when tracking register defs and uses, which could potentially inhibit
these optimizations in the presence of debug information.

Reviewers: mcrosier

Subscribers: aemerson, rengolin, mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D22582

llvm-svn: 276293