[clang] [HLSL][Matrix] Update indexed matrix elements individually (PR #176216)

[Deric C. via cfe-commits]

Icohedron wrote:

> Actually one last thing I think should change, there should probably be a test that runs with a higher optimization level. Not sure if the assumption can be detected in the IR though?

I will probably just remove the code that creates the IndexAssumption. I'm not sure if any passes partaking in the lowering of HLSL even utilize it.

https://github.com/llvm/llvm-project/pull/176216