I added a fast fix in r75444 which should prevent the below cases, but won't provide minimal answers in a whole ton of cases that it ought to. Multiply needs to be rethought...

unionWith has some problems; they all take the form that the upper limit is a feasible value:

ERROR: Found invalid values!
A = 1 is in LHS = [14, 1) or
A = 1 is in RHS = [0, 8), but
A is not in (LHS union RHS) = [8, 1)
ERROR: Found invalid values!
A = 3 is in LHS = [2, 10) or
A = 3 is in RHS = [10, 1), but
A is not in (LHS union RHS) = [10, 1)
ERROR: Found invalid values!
A = 1 is in LHS = [0, 2) or
A = 1 is in RHS = [3, 0), but
A is not in (LHS union RHS) = [2, 0)
ERROR: Found invalid values!
A = 2 is in LHS = [2, 6) or
A = 2 is in RHS = [6, 0), but
A is not in (LHS union RHS) = [6, 0)
ERROR: Found invalid values!
A = 0 is in LHS = [0, 4) or
A = 0 is in RHS = [6, 0), but
A is not in (LHS union RHS) = [4, 0)
ERROR: Found invalid values!
A = 3 is in LHS = [2, 7) or
A = 3 is in RHS = [15, 1), but
A is not in (LHS union RHS) = [15, 3)

Nevermind, they don't all take that form, I looked too quickly.

intersectWith has problems, we should probably remove it and use maximalIntersectWith. Test cases:

ERROR: Found invalid values!
A = 4 is in LHS = [4, 2)
and in RHS = [6, 5), but
A is not in (LHS intersect RHS) = [6, 2)
ERROR: Found invalid values!
A = 4 is in LHS = [4, 2)
and in RHS = [6, 5), but
A is not in (LHS intersect RHS) = [6, 2)
ERROR: Found invalid values!
A = 10 is in LHS = [8, 5)
and in RHS = [12, 11), but
A is not in (LHS intersect RHS) = [12, 5)
ERROR: Found invalid values!
A = 4 is in LHS = [4, 2)
and in RHS = [6, 5), but
A is not in (LHS intersect RHS) = [6, 2)
ERROR: Found invalid values!
A = 5 is in LHS = [7, 6)
and in RHS = [4, 3), but
A is not in (LHS intersect RHS) = [7, 3)
ERROR: Found invalid values!
A = 4 is in LHS = [7, 5)
and in RHS = [4, 0), but
A is not in (LHS intersect RHS) = [7, 0)
ERROR: Found invalid values!
A = 5 is in LHS = [7, 6)
and in RHS = [4, 3), but
A is not in (LHS intersect RHS) = [7, 3)
ERROR: Found invalid values!
A = 5 is in LHS = [7, 6)
and in RHS = [4, 3), but
A is not in (LHS intersect RHS) = [7, 3)
ERROR: Found invalid values!
A = 4 is in LHS = [7, 5)
and in RHS = [4, 0), but
A is not in (LHS intersect RHS) = [7, 0)
ERROR: Found invalid values!
A = 4 is in LHS = [7, 5)
and in RHS = [4, 0), but
A is not in (LHS intersect RHS) = [7, 0)

Keep in mind all examples are computed with 4 bits.

Here is a signExtend failure:

ERROR: Found invalid values!
A = 7 is in LHS = [15, 15), and
A.sext(26) = 7 is not in LHS.signExtend(26) = [67108856, 7)

And two sgt errors:

ERROR: Found invalid values!
A = 7 and LHS = [7, 0), but
A sgt (LHS.getSignedMax() = 15)
ERROR: Found invalid values!
A = 7 and LHS = [7, 6), but
A sgt (LHS.getSignedMax() = 5)

klee driver for testing ConstantRange

This has been fixed somewhere along the way.
I can confirm that it I can reproduce it with the version before r75444, but with the current release (LLVM 10.0) it is not reproducible (all tests pass, including the attached test case).