KT-10139: any if without else used in expression is an error

regardless of expected type: it can't be an expression, even of type Unit.

 #KT-10139 Fixed

compiler/testData/diagnostics/tests/functionLiterals/return/IfWithoutElse.kt

@@ -2,7 +2,7 @@ val flag = true
 
 // type of a was checked by txt
 val a/*: () -> Any*/ = l@ {
-    if (flag) return@l 4
+    <!INVALID_IF_AS_EXPRESSION!>if (flag) return@l 4<!>
 }
 
 val b/*: () -> Int */ = l@ {

compiler/testData/diagnostics/tests/functionLiterals/return/IfWithoutElseWithExplicitType.kt

@@ -1,7 +1,7 @@
 val flag = true
 
 val a: () -> Int = l@ {
-    <!TYPE_MISMATCH!>if (flag) return@l 4<!>
+    <!TYPE_MISMATCH, INVALID_IF_AS_EXPRESSION!>if (flag) return@l 4<!>
 }
 
 val b: () -> Unit = l@ {
@@ -9,7 +9,7 @@ val b: () -> Unit = l@ {
 }
 
 val c: () -> Any = l@ {
-    if (flag) return@l 4
+    <!INVALID_IF_AS_EXPRESSION!>if (flag) return@l 4<!>
 }
 
 val d: () -> Int = l@ {