[HLSL] Add WaveMultiPrefixCountBits (#238132)
Add `WaveMultiPrefixCountBits`, with DirectX-only lowering.
This PR does not add SPIR-V support. SPIR-V calls report that the
builtin is not supported on the target.
Fixes #99211
Assisted by: Github Copilot
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 40997c2f-5aaa-41c6-8d1f-3e2e9ada315f
