Fix lvalue output parameter assignment.

Fix lvalue output parameter assignment.

Output parameters can be any kind of lvalue, such as struct members or
array elements.

Change-Id: I4dce9dddfa95f58275fe3f6a4acf55532330a8f7
Reviewed-on: https://swiftshader-review.googlesource.com/5570
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>