Support 'when' with subject variable in psi2ir

compiler/testData/ir/irText/expressions/whenWithSubjectVariable.kt

@@ -0,0 +1,13 @@
+// !LANGUAGE: +VariableDeclarationInWhenSubject
+
+fun foo(): Any = 1
+
+fun test() =
+    when (val y = foo()) {
+        42 -> 1
+        is String -> y.length
+        !is Int -> 2
+        in 0..10 -> 3
+        !in 10..20 -> 4
+        else -> -1
+    }