[llvm] [AMDGPU] Extend ComputePGMRSrc3 to gfx10+. NFCI. (PR #129289)

[Stanislav Mekhanoshin via llvm-commits]

rampitec wrote:

Since https://github.com/llvm/llvm-project/pull/126981 review has stalled, I have factored out just infrastructure part of it.

https://github.com/llvm/llvm-project/pull/129289