Fix for PR#330.

When looking at getelementptr instructions, make sure to use a forwarded
type.  We want to do this because a DerivedType may drop its uses and then
refine its users, who may then use another user who hasn't been refined yet.
By getting the forwarded type, we always ensure that we're looking at a
Type that isn't in a halfway refined state.

Now, I should be able to put this stuff in PATypeHandle, but it doesn't work
for some reason.  This should do for now.

llvm-svn: 13386