cty: Treat unrefined numeric ranges as infinities

Previously we were treating these as unknown, which is also a reasonable
way to model a lack of bounds but is less convenient when we want to do
arithmetic against the bounds.

cty/unknown_refinement.go

@@ -246,8 +246,10 @@ func (b *RefinementBuilder) NumberRangeLowerBound(min Value, inclusive bool) *Re
 		}
 	}
 
-	wip.min = min
-	wip.minInc = inclusive
+	if min != NegativeInfinity {
+		wip.min = min
+		wip.minInc = inclusive
+	}
 
 	wip.assertConsistentBounds()
 	return b
@@ -293,8 +295,10 @@ func (b *RefinementBuilder) NumberRangeUpperBound(max Value, inclusive bool) *Re
 		}
 	}
 
-	wip.max = max
-	wip.maxInc = inclusive
+	if max != PositiveInfinity {
+		wip.max = max
+		wip.maxInc = inclusive
+	}
 
 	wip.assertConsistentBounds()
 	return b

cty/value_range.go

@@ -137,6 +137,9 @@ func (r ValueRange) NumberLowerBound() (min Value, inclusive bool) {
 		panic(fmt.Sprintf("NumberLowerBound for %#v", r.ty))
 	}
 	if rfn, ok := r.raw.(*refinementNumber); ok && rfn.min != NilVal {
+		if !rfn.min.IsKnown() {
+			return NegativeInfinity, true
+		}
 		return rfn.min, rfn.minInc
 	}
 	return UnknownVal(Number), false
@@ -159,6 +162,9 @@ func (r ValueRange) NumberUpperBound() (max Value, inclusive bool) {
 		panic(fmt.Sprintf("NumberUpperBound for %#v", r.ty))
 	}
 	if rfn, ok := r.raw.(*refinementNumber); ok && rfn.max != NilVal {
+		if !rfn.max.IsKnown() {
+			return PositiveInfinity, true
+		}
 		return rfn.max, rfn.maxInc
 	}
 	return UnknownVal(Number), false