[llvm] Add tests for upcoming fixes to pointer/index type confusion.

Various parts of the codebase are using getIntPtrType() and its
relatives when getting the type of the offset argument to GEP. Most
such code has been updated to use the pointer index type field from
the data layout, but there is code that still assumes these two types
are the same in certain optimizaiton passes.

This commit adds regression tests to capture the old behavior.

Reviewed By: #amdgpu, arsenm

Differential Revision: https://reviews.llvm.org/D143435