[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

compiler/testData/debug/stepping/defaultParameter.kt

@@ -27,9 +27,7 @@ fun box() {
 // test.kt:11 box
 // test.kt:3 <init>
 // test.kt:11 box
+// test.kt:6 foo$default
 // test.kt:4 computeParam
-// test.kt:6 foo$default
-// test.kt:6 foo$default
 // test.kt:7 foo
-// test.kt:6 foo$default
-// test.kt:12 box
+// test.kt:12 box