Make smart casts work with variable declared in when subject

compiler/testData/diagnostics/tests/when/withSubjectVariable/smartCastsOnSubjectVariable.kt

@@ -0,0 +1,8 @@
+// !LANGUAGE: +VariableDeclarationInWhenSubject
+
+fun test(x: Any?) =
+        when (val y = x) {
+            is String -> "String, length = ${<!DEBUG_INFO_SMARTCAST!>y<!>.length}"
+            null -> "Null"
+            else -> "Any, hashCode = ${<!DEBUG_INFO_SMARTCAST!>y<!>.hashCode()}"
+        }

compiler/testData/diagnostics/tests/when/withSubjectVariable/smartCastsOnSubjectVariable.txt

@@ -0,0 +1,3 @@
+package
+
+public fun test(/*0*/ x: kotlin.Any?): kotlin.String