[APInt] Add a concat method, use LLVM_UNLIKELY to help optimizer.

Three unrelated changes:

1) Add a concat method as a convenience to help write bitvector
   use cases in a nicer way.
2) Use LLVM_UNLIKELY as suggested by @xbolva00 in a previous patch.
3) Fix casing of some "slow" methods to follow naming standards.

Differential Revision: https://reviews.llvm.org/D109620