optimization/10080: Loop unroller nearly useless

[rakdver@atrey.karlin.mff.cuni.cz]

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=
10080

the problem is that ++i is translated into

(insn 28 26 29 1 0x40135c40 (set (reg:DI 79)
        (plus:DI (reg/v:DI 72 [ i ])
            (const_int 1 [0x1]))) -1 (nil)
    (nil))
(insn 29 28 75 1 0x40135c40 (set (reg/v:DI 72 [ i ])
        (sign_extend:DI (subreg:SI (reg:DI 79) 0))) -1 (nil)
    (nil))

but my overly simplistic analysis does not recognize it.
I am workning on fix.

Zdenek