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/ir/ir.psi2ir/src/org/jetbrains/kotlin/psi2ir/generators/BranchingExpressionGenerator.kt

@@ -16,6 +16,7 @@
 
 package org.jetbrains.kotlin.psi2ir.generators
 
+import org.jetbrains.kotlin.builtins.KotlinBuiltIns
 import org.jetbrains.kotlin.ir.UNDEFINED_OFFSET
 import org.jetbrains.kotlin.ir.builders.whenComma
 import org.jetbrains.kotlin.ir.declarations.IrVariable
@@ -85,12 +86,17 @@ class BranchingExpressionGenerator(statementGenerator: StatementGenerator) : Sta
             scope.createTemporaryVariable(statementGenerator.generateExpression(it), "subject")
         }
 
+
+        val inferredType = getInferredTypeWithImplicitCastsOrFail(expression)
+
         // TODO relies on ControlFlowInformationProvider, get rid of it
-        val resultType =
-                if (context.bindingContext[BindingContext.USED_AS_EXPRESSION, expression] ?: false)
-                    getInferredTypeWithImplicitCastsOrFail(expression)
-                else
-                    context.builtIns.unitType
+        val isUsedAsExpression = context.bindingContext[BindingContext.USED_AS_EXPRESSION, expression] ?: false
+
+        val resultType = when {
+            isUsedAsExpression -> inferredType
+            KotlinBuiltIns.isNothing(inferredType) -> inferredType
+            else -> context.builtIns.unitType
+        }
 
         val irWhen = IrWhenImpl(expression.startOffset, expression.endOffset, resultType, IrStatementOrigin.WHEN)