simplify -1 and I handling in Mul.flatten

The best of 3 runs of the core test suite gave, in seconds,

master: 43, 48, 57
now: 36, 36, 45

So this appears to be a little faster and leads to a more compact result.
watch for unevaluated I**e
Merge pull request #1987 from smichr/n1

-1 and I flatten better in Mul.flatten