[GlobalISel][AArch64] Select llvm.bswap* for non-vector types

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.

Differential Revision: https://reviews.llvm.org/D58081

llvm-svn: 353861