Tracking issue for reversing the bit pattern in an integer

[Amanieu]

Tracking issue for the reverse_bits method implemented in #48573.

[SimonSapin]

Motivation from #32798 (comment)

On ARM, there are dedicated instructions for reversing bits. The intrinsic would lower to a single RBIT instruction.

[zbentley]

I'm a user that would benefit from this being stabilized. No worries if I have to wait until an LLVM requirement version bump, though. We would benefit from it, but aren't in a position to contribute support, so we are happy to wait/offer testing or documentation help.

[raphaelcohn]

Just to add to this, these intrinsics are useful for formatting data that is represented bit reversed, such as the IEEE standard for bit-reversed MACs, token ring addresses, etc.