opposite to CreateStructGEP

Does getelementptr have a counterpart function which unindexes a
pointer? In particular, I'm using CreateStructGEP from the builder and
want to get back to the struct pointer from a member pointer.

fptr = b.CreateStructGEP( sptr, 2 );

//I want to do this
sptr = b.CreateUnStructGEP( fptr, sptr_type, 2 );

I know I can calculate the offsets on my own, but it'd be nice to not
have to since GEP already does it in the forward direction.