use isZero()

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208614 91177308-0d34-0410-b5e6-96231b3b80d8
do not assert when delinearization fails

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208615 91177308-0d34-0410-b5e6-96231b3b80d8