ConstraintAnalysis: Handle cases with extra constraints

[kripken]

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.

These are hard to test separately. While doing so I also fixed two other
issues (which I could split out, but again, full testing only really happens
together):

• 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.