Do compare constant SCEV values in SCEVComplexityCompare, because

Do compare constant SCEV values in SCEVComplexityCompare, because
even though the order doesn't matter at the top level of an expression,
it does matter when the constant is a subexpression of an n-ary
expression, because n-ary expressions are sorted lexicographically.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@73358 91177308-0d34-0410-b5e6-96231b3b80d8