peephole2: dead regs not marked as dead

[Georg Lay]

Hi, I just came across an optimization issue in pass peephole2:

Regs that are "naturally" dead because the function ends are not marked as dead,
and therefore some optimization opportunities pass by unnoticed, e.g. together
with recog.c::peep2_reg_dead_p() et. al.

As I could not find a related PR, is this worth opening one?

Regards, G.Lay

Ah, I am using version 4.5.1