This teaches the IRTranslator to emit G_BSWAP when it runs into Intrinsic::bswap. This allows us to select G_BSWAP for non-vector types in AArch64.
Add a select-bswap.mir test, and add global isel checks to a couple existing tests in test/CodeGen/AArch64.
This doesn't handle every bswap case, since some of these rely on known bits stuff. This just lets us handle the naive case.