Support 'when' with subject variable in JVM BE

compiler/testData/codegen/box/when/whenSubjectVariable/rangeCheckOnSubjectVariable.kt

@@ -0,0 +1,9 @@
+// !LANGUAGE: +VariableDeclarationInWhenSubject
+
+val x = 1
+
+fun box() =
+    when (val y = x) {
+        in 0..2 -> "OK"
+        else -> "Fail: $y"
+    }