New target SDNodes are added: AArch64ISD::MOPS_MEMSET, etc.
Each intrinsic is translated to one of these in SelectionDAGBuilder
A custom lowering routine for INTRINSIC_W_CHAIN is added to handle
llvm.aarch64.mops.memset.tag. This takes a separate path from the common
intrinsics but ultimately ends up in the same EmitMOPS().
This is part 4/4 of a series of patches split from
https://reviews.llvm.org/D117405 to facilitate reviewing.
Patch by Tomas Matheson, Lucas Prates and Son Tuan Vu.