[llvm] [InstSimplify] Fold `getelementptr inbounds null, idx -> null` (PR #130742)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

As we discussed on https://github.com/llvm/llvm-project/pull/130734#issuecomment-2720039103, I don't want to add a dependency between clang IRGen and LLVM constant folding.

I'm fine with extending AST checking for "null pointer" patterns.

https://github.com/llvm/llvm-project/pull/130742