[Bug target/89794] combine incorrectly forwards register value through auto-inc operation

[rearnsha at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89794

--- Comment #6 from Richard Earnshaw <rearnsha at gcc dot gnu.org> ---
There seems to be more to this than initially thought.  Another insn is in
play.

(insn 12 10 14 2 (set (reg:SI 129)
        (bswap:SI (subreg:SI (reg:DI 127 [ i ]) 4))) "/tmp/test3.c":10:7 331
{*arm_rev}
     (expr_list:REG_DEAD (reg:DI 127 [ i ])
        (nil)))

Which uses the value loaded by the pre-modify instruction.

Combine manages to combine (and simplify insns 10 and 12, but the
simplification is to

(set (reg:SI 129) (const_int 0))

and we've lost the pre-inc entirely.