Fix performance regression caused by contracts

Do not obtain all children from KtBlockExpression when you only need
the first one.
It might be crucial for a huge blocks of code

compiler/psi/src/org/jetbrains/kotlin/psi/KtBlockExpression.java

@@ -123,6 +123,11 @@ public class KtBlockExpression extends LazyParseablePsiElement implements KtElem
         return substitute != null ? substitute : super.getParent();
     }
 
+    @Nullable
+    public KtExpression getFirstStatement() {
+        return findChildByClass(KtExpression.class);
+    }
+
     @ReadOnly
     @NotNull
     public List<KtExpression> getStatements() {

compiler/psi/src/org/jetbrains/kotlin/psi/psiUtil/ktPsiUtil.kt

@@ -323,7 +323,7 @@ fun KtElement.isFirstStatement(): Boolean {
         element = parent
         parent = parent.parent
     }
-    return parent is KtBlockExpression && parent.children.first { it is KtElement } == element
+    return parent is KtBlockExpression && parent.firstStatement == element
 }