StackColoring: better handling of statically unreachable code

Summary:
Avoid assert/crash during liveness calculation in situations
where the incoming machine function has statically unreachable BBs.
Second attempt at submitting; this version of the change includes
a revised testcase.

Fixes PR37130.

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D47372

llvm-svn: 333416