[Utils] Extract EliminateUnreachableBlocks (NFC)

Summary:
Extract the functionality of eliminating unreachable basic blocks
within a function, previously encapsulated within the
-unreachableblockelim pass, and make it available as a function within
BlockUtils.h. No functional change intended other than making the logic
reusable.

Exposing this logic makes it easier to implement
https://reviews.llvm.org/D59068, which fixes coroutines bug
https://bugs.llvm.org/show_bug.cgi?id=40979.

Reviewers: mkazantsev, wmi, davidxl, silvas, davide

Reviewed By: davide

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59069

llvm-svn: 355846