[opaque pointer type] Start migrating GEP creation to explicitly specify the pointee...

[opaque pointer type] Start migrating GEP creation to explicitly specify the pointee type

I'm just going to migrate these in a pretty ad-hoc & incremental way -
providing the backwards compatible API for now, then locally removing
it, fixing a few callers, adding it back in and commiting those callers.
Rinse, repeat.

The assertions should ensure that if I get this wrong we'll find out
about it and not just have one giant patch to revert, recommit, revert,
recommit, etc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232240 91177308-0d34-0410-b5e6-96231b3b80d8