[clang] [Matrix] Preserve signedness when extending matrix index expression. (PR #103044)

[Florian Hahn via cfe-commits]

fhahn wrote:

> Do you just want whatever the logic is for array subscripts? I believe array subscripts technically leave the index unconverted, and then we perform the necessary conversion when emitting IR.

Updated to do it during codegen. Unfortunately it needs to be done in 2 different places it seems. Is this what you had in mind?

https://github.com/llvm/llvm-project/pull/103044