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/codegen/box/constants/constantsInWhen.kt

@@ -1,7 +1,3 @@
-// Even before any IR lowerings, the type of `when` is determined to be
-// Unit even though the outer `if` still returns `Int?`. This results
-// in a ClassCastException when that Unit is converted into a Number.
-// IGNORE_BACKEND: JVM_IR
 fun test(
         b: Boolean,
         i: Int