Perl_op_lvalue_flags(): skip OPf_WANT_VOID ops.

Perl_op_lvalue_flags(): skip OPf_WANT_VOID ops.

Currently this function asserts that its 'o' argument is non-VOID;
later when recursing an OP_LIST, it skips any kids which are VOID.

This commit changes it so that the assert becomes a return, and
OP_LIST doesn't check whether its kids are VOID.

Doing it this way makes it easier to shortly make Perl_op_lvalue_flags()
non-recursive.

The only functional difference is that on debugging builds,
Perl_op_lvalue_flags() will no longer fail an assert if inadvertently
called with a VOID op.