tcg: Add TCG_OPF_BB_EXIT

tcg: Add TCG_OPF_BB_EXIT

Use this to notice the opcodes that exit the TB, which implies
that local temps are really dead and need not be synced.

Previously we so marked the true end of the TB, but that was
immediately overwritten by the la_bb_end invoked by any
TCG_OPF_BB_END opcode, like exit_tb.

Reviewed-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>