Reassociate: cannot negate a INT_MIN value

Summary:
When trying to canonicalize negative constants out of
multiplication expressions, we need to check that the
constant is not INT_MIN which cannot be negated.

Reviewers: mcrosier

Reviewed By: mcrosier

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D7286

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 228872