[SimplifyCFG] Added condition assumption for unreachable blocks

Summary: PR41688

Reviewers: spatel, efriedma, craig.topper, hfinkel, reames

Reviewed By: hfinkel

Subscribers: javed.absar, dmgreen, fhahn, hfinkel, reames, nikic, lebedev.ri, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61409

llvm-svn: 361707