tcg: mark set_label with TCG_OPF_BB_END flag

tcg: mark set_label with TCG_OPF_BB_END flag

set_label is effectively the end of a basic block, as no optimization
can be made accross it. It was treated as such in the liveness analysis
code, but as a special case.

Mark it with TCG_OPF_BB_END flag so that this information can be used
by other parts of the TCG code, and remove the special case in the liveness
analysis code.

Cc: Blue Swirl <blauwirbel@gmail.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>