change#3728 was flawed (loop contexts saw the wrong statement

change#3728 was flawed (loop contexts saw the wrong statement
info, causing loop control constructs to not find the label);
disable OP_SETSTATE entirely and add a fix that is specifically
targetted at disabling the OP_LINESEQ optimization in else BLOCK,
which was what the original patch was supposed to fix

TODO: remove the remainder of the setstate logic if it can't
be used anywhere else (it isn't used anywhere now)

p4raw-link: @3728 on //depot/perl: 7399586d384137f7ae66bcc82a83b0df7dd429e5

p4raw-id: //depot/perl@4309