InstCombine: Optimize -x s< cst

Summary:
This gets rid of a sub instruction by moving the negation to the
constant when valid.

Reviewers: nicholas

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D3773

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208827 91177308-0d34-0410-b5e6-96231b3b80d8