[clang] [llvm] [AMDGPU] Adding the amdgpu-num-work-groups function attribute (PR #79035)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Oh, yeah, agreed that known-bits data is marginal ... but a big pile of marginal improvements stacks up.

The main benefit would be is if we can reduce 64-bit indexing calculations down to 32-bit. Not sure if that's implemented anywhere 

https://github.com/llvm/llvm-project/pull/79035