[AArch64] Merge LDRSWpre-LD[U]RSW pair into LDPSWpre

This patch optimizes a pair of LDRSWpre and LDRSWui (or LDURSWi)
instructions into a single LDPSWpre instruction.  This is a missing case
in D99272.

MIR test cases in D152564 are updated to verify the optimization.

Differential Revision: https://reviews.llvm.org/D152407