fix constraints merging of non-constant bounds

Before, something like `read_int("x", 0, n)` would print the minimum
seen value for `n` in the output range for x, instead of the maximum
value.
That leads to confusing messages such as:
0 <= x <= 2
WARNING: upper bound not reached. max value 999.
or even
1000 <= x <= 0

Author: RagnarGrootKoerkamp

bin/validate.py

@@ -53,9 +53,9 @@ def _merge_constraints(constraints_path: Path, constraints: ConstraintsDict):
                     vmin = c[3]
                 if c[4] > vmax:
                     vmax = c[4]
-                if c[5] > low:
+                if c[5] < low:
                     low = c[5]
-                if c[6] < high:
+                if c[6] > high:
                     high = c[6]
             constraints[loc] = (name, has_low, has_high, vmin, vmax, low, high)