Restricting build_*_gep to make it safer.

[JCBurnside]

Is your feature request related to a problem? Please describe.
I have multiple times accidentally used build_*_gep with the wrong pointee type thinking it would the result type in llvm15+

Describe the solution you'd like
Add a new trait potentially called IndexableType or GEPable that would restrict what types can be used in a gep instruction. it would be implemented on PointerType, VectorType, ArrayType, and StructType

Describe possible drawbacks to your solution
potentially breaks backwards compat.

Describe alternatives you've considered
paying more attention when writing gep instructions.

[JCBurnside]

Seems like a simple marker trait to help limit the unsafeness of the build gep calls.