Revert r300657 due to crashes in stage2 of bootstraps:

http://lab.llvm.org:8011/builders/clang-with-lto-ubuntu/builds/2476/steps/build-stage2-LLVMgold.so/logs/stdio
http://bb.pgr.jp/builders/clang-3stage-x86_64-linux/builds/15036/steps/build_llvmclang/logs/stdio

I've updated the commit thread, reverting to get the bots back to green.

Original commit summary:
[JumpThread] We want to fold (not thread) when all predecessor go to single BB's successor.

llvm-svn: 300662