[AArch64] Fix ldst optimization of non-immediate store offset

When matching store instruction for ldst opt, we should make sure store instr is in 'reg+imm' form as load instr,
otherwise, it will have assertion in isLdOffsetInRangeOfSt since it will use getImm() directly.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D87905