Revert "[BasicAA] Revert "Revert r218714 - Make better use of zext and sign information.""

This reverts commit r219135 -- still causing miscompiles in SPEC it seems...

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219432 91177308-0d34-0410-b5e6-96231b3b80d8
Remove a compiler bug workaround from 2007. The affected versions of gcc are long gone.

NFC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219433 91177308-0d34-0410-b5e6-96231b3b80d8
[Reassociate] Don't canonicalize X - undef to X + (-undef).

Phabricator Revision: http://reviews.llvm.org/D5674
PR21205

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219434 91177308-0d34-0410-b5e6-96231b3b80d8