ConstraintAnalysis: Optimize loops

[kripken]

One Weird Trick is enough: eagerly extend ranges of constants:

x = C
branch on x < D where C < D
=>
x >= C && x < D

This is simpler than the typical approach used in Abstract Interpretation,
as we do it eagerly (immediately on a branch). This eagerness might
lose some precision, but not in cases we care about, I don't think:
if x is a constant and we branch on it, then it must be a loop variable
that will increment (if it isn't in a loop, the constant x would have been
propagated to the branch by other passes).

[tlively]

Nice!

I'm concerned we'll still need a more conservative fallback, though. Consider this loop:

x = 0
y = 100
while (x < y) {
  ++x;
  ++x;
  ++y;
}

This loop will also execute 100 times, and hopefully we are able to analyze all the individual operations, but the loop condition does not contain a constant, so this particular widening heuristic will not apply.

[tlively]

Probably worth a comment about why we only handle the case where there is only a single constraint.

It also might be easier to read if we invert the conditions and early return false when our expectations are not met.