Improve JetNameValidatorImpl performance

idea/testData/refactoring/introduceVariable/noConflictWithInnerVariable.kt

@@ -0,0 +1,7 @@
+fun foo() {
+    val a = 1
+    if (<selection>a > 0</selection>) {
+        val b = 0
+        println(b)
+    }
+}

idea/testData/refactoring/introduceVariable/noConflictWithInnerVariable.kt.after

@@ -0,0 +1,8 @@
+fun foo() {
+    val a = 1
+    val b1 = a > 0
+    if (b1) {
+        val b = 0
+        println(b)
+    }
+}