[llvm] [AMDGPU] Add user flag to set min/max number of AGPRs. (PR #216800)

[via llvm-commits]

carlobertolli wrote:

@arsenm I updated the PR with an alternative approach: set AGPR use to 0 when occupancy is >=2. No user flag.
Would this be a better approach?
This affects 100 lit tests, which I did not update yet - hence draft mode.

https://github.com/llvm/llvm-project/pull/216800