Don’t use OP_SIBLING(o) in op.c:ck_sassign

When this is called, the op is not part of a larger tree yet, so its
sibling is always NULL.
op.c:ck_sassign: Simplify freeing of sassign op

Instead of detaching both the rhs and lhs and then freeing the assign-
ment and the lhs, just detach the rhs and free the assignment.  This
results in smaller machine code.
op.c:scalarvoid: Remove redundant scalar(o)