[AA FIR] KtFirReferenceShortener: process only statements in scripts

We shouldn't process other elements (like parameters/receivers) because
we can't modify them anyway. This will help to avoid exceptions like
```
Attempt to load text for binary file which doesn't have a decompiler plugged in
```
in tests

^KT-61431

analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/components/KtFirReferenceShortener.kt

@@ -382,6 +382,12 @@ private class ElementsToShortenCollector(
         }
     }
 
+    override fun visitScript(script: FirScript) {
+        script.statements.forEach {
+            it.accept(this)
+        }
+    }
+
     override fun visitElement(element: FirElement) {
         element.acceptChildren(this)
     }