Refix a use of explicit pointer types in GEP constant folding

In the glorious future of opaque pointer types, it won't be possible to
retrieve the pointee type of a pointer type which is what's being done
in this GEP loop - but the first iteration is always a pointer type and
the loop doesn't care about that case, except whether or not the index
is a constant.

So pull that special case out before the loop and start at the second
iteration (index 1) instead.

Originally committed in r236670 and reverted with a test case in
r239015. This change keeps the test case working while also avoiding
depending on pointee types.

llvm-svn: 239629