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/irText/expressions/whenReturn.txt

@@ -2,10 +2,10 @@ FILE /whenReturn.kt
   FUN public fun toString(grade: kotlin.String): kotlin.String
     VALUE_PARAMETER value-parameter grade: kotlin.String
     BLOCK_BODY
-      BLOCK type=kotlin.Unit origin=WHEN
+      BLOCK type=kotlin.Nothing origin=WHEN
         VAR IR_TEMPORARY_VARIABLE val tmp0_subject: kotlin.String
           GET_VAR 'value-parameter grade: String' type=kotlin.String origin=null
-        WHEN type=kotlin.Unit origin=WHEN
+        WHEN type=kotlin.Nothing origin=WHEN
           BRANCH
             if: CALL 'EQEQ(Any?, Any?): Boolean' type=kotlin.Boolean origin=EQEQ
               arg0: GET_VAR 'tmp0_subject: String' type=kotlin.String origin=null