revert rL349051: [DAGCombiner] after simplifying demanded elements of vector...

revert rL349051: [DAGCombiner] after simplifying demanded elements of vector operand of extract, revisit the extract

This causes an address sanitizer bot failure:
http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-fast/builds/27187/steps/check-llvm%20asan/logs/stdio

llvm-svn: 349056