Revert "[InstCombine] re-commit r218721 with fix for pr21199"

This seems to cause a miscompile when building clang, which causes a
bootstrapped clang to fail or crash in several of its tests.

See:
  http://lab.llvm.org:8013/builders/clang-x86_64-darwin11-RA/builds/1184
  http://bb.pgr.jp/builders/clang-3stage-x86_64-linux/builds/7813

This reverts commit r219282.