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/testData/ir/irText/expressions/whenUnusedExpression.kt

@@ -0,0 +1,10 @@
+// WITH_RUNTIME
+
+fun test(b: Boolean, i: Int) {
+    if (b) {
+        when (i) {
+            0 -> 1
+            else -> null
+        }
+    } else null
+}