[clang] [llvm] [IR] Add llvm.structured.gep instruction (PR #176145)

[Nathan Gauër via cfe-commits]

Keenuts wrote:

> > This needs an IR verifier check to ensure the elementtype attribute is present.
> 
> And the other preconditions, like the number of indices being correct, only constant indices for structs, etc.

Right, good point to add verifier check/tests. Added to the current PR

https://github.com/llvm/llvm-project/pull/176145