Fix containsBothSuccs and nextIsMerge

Summary: This is really what I was trying to do all along -- fix
containsBothSuccs and nextIsMerge so that they're actually correct for opcodes
other than JmpZ/NZ.  Instead of assuming imm[0].u_BA for the offset, actually
use instrJumpOffset.

Reviewed By: @ottoni

Differential Revision: D1460200