Fix arcs inserted for bytecode control flow within a region

Summary: Profile translations can contain multiple blocks. So, when inserting
arcs into a region, an arc should always go from the last block in the
source profile translation to the first block in the dest profile
translation.  While inspecting the code, I found some cases where this
wasn't the case, which are fixed here.

Reviewed By: @alexmalyshev

Differential Revision: D1512204