Remove redundant op_lvalue calls in perly.y

When (\$x)=\$y is compiled, the \ on the lhs gives lvalue context to
its argument by calling op_lvalue.  Then later the = gives lvalue con-
text to the \, calling op_lvalue again, which transforms the $x into
an lvref op (via op.c:S_lvref).

I just copied that logic when I extended aliasing via reference to
foreach \$x.  But here, we don’t need to call op_lvalue on the $x,
because we know it is going to go through op.c:S_lvref, which doesn’t
care whether it has been through op_lvalue already or not.  The end
result is the same.