Fix modulo compared to zero.

In case of modulo compared to zero, we need to do signed modulo
operation as unsigned can give different results based on whether the
dividend is negative or not.

This addresses llvm.org/PR27707

Contributed-by: Chris Jenneisch <chrisj@codeaurora.org>

Reviewers: _jdoerfert, grosser, Meinersbur

Differential Revision: http://reviews.llvm.org/D20145

llvm-svn: 271707