Revert r188119 "Kill some duplicated code for removing unreachable BBs."

It is breaking builbots with libgmalloc enabled on Mac OS X.

$ cd llvm ; mkdir release ; cd release
$ ../configure --enable-optimized —prefix=$PWD/install
$ make
$ make check
$ Release+Asserts/bin/llvm-lit -v --param use_gmalloc=1 --param \
  gmalloc_path=/usr/lib/libgmalloc.dylib \
  ../test/Instrumentation/DataFlowSanitizer/args-unreachable-bb.ll

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188142 91177308-0d34-0410-b5e6-96231b3b80d8