[AArch64] Refactoring findMatchingStore() in aarch64-ldst-opt; NFC

Summary: This change makes findMatchingStore() follow the same coding style introduced in r260275.

Reviewers: gberry, junbuml

Subscribers: aemerson, rengolin, haicheng, bmakam, mssimpso

Differential Revision: http://reviews.llvm.org/D17083

llvm-svn: 260534