[llvm] [TTI] Introduce getInstructionUniformity API for flexible uniformity analysis (PR #137639)

[Jay Foad via llvm-commits]

jayfoad wrote:

> > permlane16 is like shuffle where `src1` and `src2` are combined to form the lane select argument, so the result is uniform if either `src0` or both `src1` and `src2` are uniform.
> 
> Since the permlane16-like intrinsics only shuffle within/across 16-lane-wide row. Even the `src1` and `src2` are uniform (I think in real case they should be always a uniform mask), if `src0` is divergent, the result is still divergent (at least the values are non-uniform among different rows).

Yes, you are right of course.

https://github.com/llvm/llvm-project/pull/137639