[clang] [llvm] [InstCombine] Strip leading zero indices from GEP (PR #155415)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> I finally got some IR and maybe some other info. With `-opt-bisect-limit` it appears that the issue is coming from GVN on the function `_ZN47LlvmLibcCharacterConverterUTF32To8Test_FourByte3RunEv`. After that the particular tests fails at runtime.
> 
> I attached the IR after the pass of LCSSA which runs before GVN. Let me know how helpful that is or what I can do to help better with this issue. [after-lcssa.txt](https://github.com/user-attachments/files/22102637/after-lcssa.txt)

I don't see anything immediately problematic looking at that function, https://godbolt.org/z/34fq6Erz8. But I'm not sure overall, cc @arsenm.

https://github.com/llvm/llvm-project/pull/155415