in findGCD of multiply expr return the gcd

we used to return 1 instead of the gcd

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205800 91177308-0d34-0410-b5e6-96231b3b80d8
handle special cases when findGCD returns 1

used to fail with 'Step should divide Start with no remainder.'

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205801 91177308-0d34-0410-b5e6-96231b3b80d8
divide by the result of the gcd

used to fail with 'Step should divide Start with no remainder.'

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205802 91177308-0d34-0410-b5e6-96231b3b80d8