values within initializer blocks.
The issue occurs in code like this:
```
class C {
var b = true
init {
b = false // Missing PUTFIELD for this statement
}
}
```
Added a new statement origin for field initialization (at declaration)
instead of relying on `origin == null` in ExpressionCodegen to determine
whether to generate the initializations.
This was unintentionally broken in
d68a1898d0.
Initializers are "set field" expressions and are considered redundant
when they are:
1. In the primary constructor; and
2. Set the field to `0`, `false`, or `null`; and
3. Have a `null` origin. I.e., not in an initializer block or
constructor body, and therefore the field could not have been set by a
prior expression.