Only reduce clamp if comparison between center and max is known

Fixes #836

src/lib.rs

@@ -7704,7 +7704,7 @@ mod tests {
     );
     minify_test(
       ".foo { border-width: clamp(1em, 2em, 4vh) }",
-      ".foo{border-width:min(2em,4vh)}",
+      ".foo{border-width:clamp(1em,2em,4vh)}",
     );
     minify_test(
       ".foo { border-width: clamp(1em, 2vh, 4vh) }",
@@ -7715,6 +7715,10 @@ mod tests {
       ".foo{border-width:clamp(1px,1px + 2em,4px)}",
     );
     minify_test(".foo { border-width: clamp(1px, 2pt, 1in) }", ".foo{border-width:2pt}");
+    minify_test(
+      ".foo { width: clamp(-100px, 0px, 50% - 50vw); }",
+      ".foo{width:clamp(-100px,0px,50% - 50vw)}",
+    );
 
     minify_test(
       ".foo { top: calc(-1 * clamp(1.75rem, 8vw, 4rem)) }",

src/values/calc.rs

@@ -400,22 +400,24 @@ impl<
           None => {}
         }
 
-        let cmp = if let (Some(Calc::Value(min_val)), Calc::Value(center_val)) = (&min, &center) {
-          center_val.partial_cmp(&min_val)
-        } else {
-          None
-        };
+        if cmp.is_some() {
+          let cmp = if let (Some(Calc::Value(min_val)), Calc::Value(center_val)) = (&min, &center) {
+            center_val.partial_cmp(&min_val)
+          } else {
+            None
+          };
 
-        // If center is known to be less than the minimum, replace it with minimum and remove the min argument.
-        // Otherwise, if center is known to be greater than the minimum, remove the min argument.
-        match cmp {
-          Some(std::cmp::Ordering::Less) => {
-            center = std::mem::take(&mut min).unwrap();
-          }
-          Some(_) => {
-            min = None;
+          // If center is known to be less than the minimum, replace it with minimum and remove the min argument.
+          // Otherwise, if center is known to be greater than the minimum, remove the min argument.
+          match cmp {
+            Some(std::cmp::Ordering::Less) => {
+              center = std::mem::take(&mut min).unwrap();
+            }
+            Some(_) => {
+              min = None;
+            }
+            None => {}
           }
-          None => {}
         }
 
         // Generate clamp(), min(), max(), or value depending on which arguments are left.