[llvm] [AMDGPU] Update base addr of dyn alloca considering GrowingUp stack (PR #119822)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

Are there new unit tests for dynamic alloca somewhere? We really need those. When this code was added, it was impossible to test since clang forbade C99 variable arrays, and __builtin_alloca was broken in all the GPU languages. That should work now, so we can write tests for it.

Does this also mean the default handling path is broken for the upward stack growth direction? Should that be fixed or turned into an assert?

https://github.com/llvm/llvm-project/pull/119822