[AArch64] Improve codegen of volatile load/store of i128
Instead of generating two i64 instructions for each load or store of a
volatile i128 value (two LDRs or STRs), now emit a single LDP or STP.
Reviewers: labrinea, t.p.northover, efriedma
Reviewed By: efriedma
Subscribers: kristof.beyls, hiraditya, llvm-commits
Differential Revision: https://reviews.llvm.org/D69559