psi2ir: keep the type of when when possible

Ideally, the type of `IrWhen` should be provided by type inference for
a consistent behavior. `USED_AS_EXPRESSION` from CFG isn't always
consistent with type inference, unfortunately.

The behavior is now aligned with `if`. The type of `when` is kept when
it *can* be an expression, instead of whether it is used or not.

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

@@ -104,11 +104,9 @@ class BranchingExpressionGenerator(statementGenerator: StatementGenerator) : Sta
 
         val inferredType = getInferredTypeWithImplicitCastsOrFail(expression)
 
-        // TODO relies on ControlFlowInformationProvider, get rid of it
-        val isUsedAsExpression = get(BindingContext.USED_AS_EXPRESSION, expression) ?: false
-
         val resultType = when {
-            isUsedAsExpression -> inferredType.toIrType()
+            // Non-exhaustive when can only be used as statement.
+            expression.isExhaustiveWhen() -> inferredType.toIrType()
             KotlinBuiltIns.isNothing(inferredType) -> inferredType.toIrType()
             else -> context.irBuiltIns.unitType
         }