[opaque pointer type] The last of the GEP IRBuilder API migrations

[opaque pointer type] The last of the GEP IRBuilder API migrations

There's still lots of callers passing nullptr, of course - some because
they'll never be migrated (InstCombines for bitcasts - well they don't
make any sense when the pointer type is opaque anyway, for example) and
others that will need more engineering to pass Types around.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234126 91177308-0d34-0410-b5e6-96231b3b80d8