[JumpThreading] Refactor ThreadEdge

Summary:
This patch moves various checks from ThreadEdge to new function
TryThreadEdge The rational behind this is that I'd like to use
ThreadEdge without its checks in my upcoming patch.

This patch preserves lightweight checks as assertions in ThreadEdge.
ThreadEdge does not repeat the cost check, however.

Reviewers: wmi

Subscribers: hiraditya, jfb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70338