[ValueTracking] Teach cannotBeOrderedLessThanZeroImpl to handle vector constants.

Summary: This allows vector fabs to be removed in more cases.

Reviewers: spatel, arsenm, RKSimon

Reviewed By: spatel

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D43739

llvm-svn: 326138