[AArch64] Add tests for merging LDRSWpre-LDR pairs

This patch adds MIR test cases that test merging an LDRSWpre-LDR
instruction pair into an LDPSWpre instruction.  This optimization is
currently missing and will be added a subsequent patch (D152407), so all
test cases are no merge for now.

Differential Revision: https://reviews.llvm.org/D152564