[DA] Don't propagate from unreachable blocks

Summary: Fixes crash that could occur when a divergent terminator has an unreachable parent.

Reviewers: rampitec, nhaehnle, arsenm

Subscribers: jvesely, wdng, hiraditya, jfb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73323