Kill some duplicated code for removing unreachable BBs.

This moves removeUnreachableBlocksFromFn from SimplifyCFGPass.cpp
to Utils/Local.cpp and uses it to replace the implementation of
llvm::removeUnreachableBlocks, which appears to do a strict subset
of what removeUnreachableBlocksFromFn does.

Differential Revision: http://llvm-reviews.chandlerc.com/D1334

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188119 91177308-0d34-0410-b5e6-96231b3b80d8