ConstraintAnalysis: Handle cases with extra constraints (#8988)

1. When incrementing `x++`, drop things we can't increment, rather than
  giving up entirely.
2. When extending ranges, do not look for a single Eq constraint to a
constant - do not give up if other constraints exist aside from an Eq
one.

Also fix two issues:

* We negated operations rather than flipping them.
* We should propagate constants: if x is a constant, than any constraint
  referring to it can use that constant.