[llvm] AMDGPU: Add testcase for materializing sgpr frame indexes (PR #101306)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Why do you say not having tests? We still have the mir test. But yes, it is good to have an IR test to validate the entire pipeline.

Overall MIR is quite difficult to maintain, and easy to miss testing the real scenarios that appear.

We need end to end IR tests to validate the whole pipeline works, and to meaningfully check the final output for code quality. For example, I found at least 3 bugs in the patch by adding the IR versions of the tests. Plus I've noticed other missed optimizations 

https://github.com/llvm/llvm-project/pull/101306