Fix findPrecedingFPI

Summary: Its possible that the first fpi region contains the
region we're after. Initialize fe to nullptr, and start from 0.

Reviewed By: @jdelong

Differential Revision: D1636070
Make uniqueStubs for bindCall

Summary: Most of the information compiled into the bindCall stubs can be computed from the callee ActRec. Once thats done, we only have two stubs; one for immutable calls, and one for non-immutable calls. So make a uniqueStub for each.

Reviewed By: @edwinsmith

Differential Revision: D1633348