[ARM GlobalISel] Add support for s64 G_ADD and G_SUB.
Teach RegisterBankInfo to use the correct register class, and tell the
legalizer it's legal. Everything else just works.
The one thing that's slightly weird about this compared to SelectionDAG
isel is that legalization can't distinguish between i64 and <1 x i64>,
so we might end up with more NEON instructions than the user expects.
Differential Revision: https://reviews.llvm.org/D63585