[libcu++] Optimize cuda::std::rotl and cuda::std::rotr

[davebayer]

This PR optimizes cuda::std::rotl and cuda::std::rotr for 8, 16, 64 and 128-bit wide unsigned integers.

Here is a before/after table of number of generated SASS instructions on sm120:

Type	Before	After	Notes
uint8_t	32	4	Repeats lowest 8-bits in 32-bit word and performs funnel shift.
uint16_t	33	4	Repeats lowest 16-bits in 32-bit word and performs funnel shift.
uint64_t	26	5	Performs 2 funnel shifts and shuffles the 32-bit words if needed.
__uint128_t	40	13	Performs 4 funnel shifts and shuffles the 32-bit words if needed. Most of the instructions are SEL.

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Reviewing files that changed from the base of the PR and between a262443 and a883b8b.

📒 Files selected for processing (1)

• libcudacxx/include/cuda/std/__bit/rotate.h

🚧 Files skipped from review as they are similar to previous changes (1)

• libcudacxx/include/cuda/std/__bit/rotate.h

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📝 Walkthrough

Summary by CodeRabbit

• Performance Improvements
  • Improved device-side bit-rotation performance for unsigned 8-bit, 16-bit, 32-bit, and 64-bit values.
  • Added specialized optimized paths for 8/16 using byte-based operations and for 64-bit by rotating low/high 32-bit halves and recombining, while keeping existing behavior consistent across supported types.

Walkthrough

Changes

Device rotate paths

Layer / File(s)	Summary
Specialized device rotate implementations libcudacxx/include/cuda/std/__bit/rotate.h	Device rotr and rotl paths add uint8_t and uint16_t handling through __byte_perm and 32-bit funnelshifts, plus uint64_t handling through split-half funnelshifts and conditional recombination.

Possibly related PRs

• NVIDIA/cccl#9874: Updates 64-bit rotate builtin macro definitions in the same header.

Suggested reviewers: wmaxey

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[github-actions]

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