Do not throw an exception when a when-statement with no else makes no match

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

@@ -0,0 +1,9 @@
+fun box(): String {
+    var r = "OK"
+    val x = 3
+    val y: Unit = when (x) {
+        1 -> { r = "Fail 0" }
+        2 -> { r = "Fail 1" }
+    }
+    return r
+}

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

@@ -0,0 +1,9 @@
+enum class En {
+    A
+    B
+}
+
+fun box(): String = when(En.A) {
+    En.A -> "OK"
+    En.B -> "Fail 1"
+}

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

@@ -0,0 +1,12 @@
+enum class En {
+    A
+    B
+}
+
+fun box(): String {
+    when(En.A) {
+        En.A -> "s1"
+        En.B -> "s2"
+    }
+    return "OK"
+}

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

@@ -0,0 +1,14 @@
+enum class En {
+    A
+    B
+}
+
+fun box(): String {
+
+  val u: Unit = when(En.A) {
+    En.A -> {}
+    En.B -> {}
+  }
+
+  return "OK"
+}

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

@@ -0,0 +1,11 @@
+var r = "OK"
+
+fun foo(x: Int) {
+    return when (x) {
+        1 -> { r = "Fail" }
+    }
+}
+fun box(): String {
+    foo(0)
+    return r
+}

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

@@ -0,0 +1,8 @@
+fun box(): String {
+    val x = 3
+    when (x) {
+        1 -> {}
+        2 -> {}
+    }
+    return "OK"
+}