[JS IR] Improve debug info for functions with default parameters

Don't generate unnecessary mappings so that the user doesn't need to
step in many times to get where they want

#KT-46276

js/js.translator/testData/lineNumbers/optionalArgs.kt

@@ -7,5 +7,5 @@ fun box(
     println(y)
 }
 
-// LINES(JS):    1 8 2 2 2 2     3 3 3 4   6 6 7 7
-// LINES(JS_IR): 1 1 2       1 1       4 1 6 6 7 7
+// LINES(JS):    1 8 2 2 2 2     3 3 3 4 6 6 7 7
+// LINES(JS_IR): 1   2                 4 6 6 7 7