[llvm] [Support][APint] Optimize fallback case in APInt::reverseBits (PR #189291)

[Max Graey via llvm-commits]

MaxGraey wrote:

I'm a little confused. After all, `isSingleWord` essentially checks that `BitWidth <= 64`, which is exactly what the dedicated special cases via switch-block at the beginning.

https://github.com/llvm/llvm-project/pull/189291