[opaque pointer type] more gep API migrations

[opaque pointer type] more gep API migrations

Adding nullptr to all the IRBuilder stuff because it's the first thing
that fails to build when testing without the back-compat functions, so
I'll keep having to re-add these locally for each chunk of migration I
do. Might as well check them in to save me the churn. Eventually I'll
have to migrate these too, but I'm going breadth-first.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232270 91177308-0d34-0410-b5e6-96231b3b80d8