Fix CFG problem for 'when' in Konan

If type of 'when' expression is 'Nothing', it should be kept that way
even if the expression itself is implicitly coerced to Unit.

compiler/testData/ir/irCfg/when/whenReturn.txt

@@ -5,7 +5,7 @@ CONTENT
       1 FUN public fun toString(grade: kotlin.String): kotlin.String
       2 GET_VAR 'value-parameter grade: String' type=kotlin.String origin=null
       3 VAR IR_TEMPORARY_VARIABLE val tmp0_subject: kotlin.String
-      4 WHEN type=kotlin.Unit origin=WHEN
+      4 WHEN type=kotlin.Nothing origin=WHEN
       5 GET_VAR 'tmp0_subject: String' type=kotlin.String origin=null
       6 CONST String type=kotlin.String value='A'
 OUTGOING -> BB 1, 5