[AArch64LoadStoreOpt] Handle offsets correctly for post-indexed paired loads.

Trunk would try to create something like "stp x9, x8, [x0], #512", which isn't actually a valid instruction.

Differential revision: https://reviews.llvm.org/D23368



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@278559 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed