[AArch64][GlobalISel] Mark some G_BITREVERSE types as legal + select them
We fall back on G_CTTZ_ZERO_UNDEF a lot when building clang for arm64 with
Handling this will require that we can handle G_BITREVERSE.
This patch marks G_BITREVERSE instructions with natively supported types as
legal. We get selection on these types for free via the importer.
Differential Revision: https://reviews.llvm.org/D103999