[JumpThreading] A bug fix for stale loop info after unfold select

Summary:
The return value of a TryToUnfoldSelect call was not checked, which led to an
incorrectly preserved loop info and some crash.

The original crash was reported on https://reviews.llvm.org/D59514.

Reviewers: davidxl, amehsan

Reviewed By: davidxl

Subscribers: fhahn, brzycki, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61920

llvm-svn: 360780