InstCombine: try to transform A-B < 0 into A < B

We are allowed to move the 'B' to the right hand side if we an prove
there is no signed overflow and if the comparison itself is signed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225034 91177308-0d34-0410-b5e6-96231b3b80d8