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).