[AArch64] Don't form paired loads from epilogue operations on Windows

AArch64LoadStoreOptimizer has a bunch of different guards to avoid
corrupting Windows SEH prologues/epilogues, but apparently we missed the
case of merging two instructions where the first instruction isn't part
of the epilogue, but the second instruction is.

Fixes issue discovered at https://reviews.llvm.org/D130049#3704064

Differential Revision: https://reviews.llvm.org/D134992