KT-16437 Incorrect type inference for some when coerced to Unit

If the result of 'when' is not used in an expression,
this 'when' expression has type 'Unit' despite of whatever FE has inferred.

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

@@ -85,7 +85,12 @@ class BranchingExpressionGenerator(statementGenerator: StatementGenerator) : Sta
             scope.createTemporaryVariable(statementGenerator.generateExpression(it), "subject")
         }
 
-        val resultType = 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 irWhen = IrWhenImpl(expression.startOffset, expression.endOffset, resultType, IrStatementOrigin.WHEN)