Exhaustive when with 'Unit' result now also generates an exception in else branch #KT-12192 Fixed

compiler/testData/codegen/box/when/implicitExhaustiveAndReturn.kt

@@ -0,0 +1,9 @@
+fun test(i: Int): String {
+    when (i) {
+        0 -> return "0"
+        1 -> return "1"
+    }
+    return "OK"
+}
+
+fun box(): String = test(42)

compiler/testData/codegen/bytecodeText/when/exhaustiveWhenUnit.kt

@@ -0,0 +1,18 @@
+enum class AccessMode { READ, WRITE, EXECUTE }
+
+fun whenExpr(access: AccessMode) {
+    return when (access) {
+        AccessMode.READ -> {}
+        AccessMode.WRITE -> {}
+        AccessMode.EXECUTE -> {}
+    }
+}
+
+fun box(): String {
+    whenExpr(AccessMode.EXECUTE)
+    return "OK"
+}
+
+// 1 TABLESWITCH
+// 0 LOOKUPSWITCH
+// 1 ATHROW