Possible bug in cse.c affecting pre/post-modify mem access

["A. Skrobov" <tyomitch at gmail dot com>]

Hello,

While working on a port of GCC for a non-public architecture that has
pre/post-modify memory access instructions, I discovered what looks
like a bug which can cause incorrect code generation.

My suggested fix is trivial:
https://github.com/tyomitch/gcc/commit/7d9cc102adf11065358d4694109ce3e9f0b5c642
-- but I cannot submit this patch without a testcase, and my expertise
in the standard GCC target architectures is insufficient for
reproducing this bug in any one of them. So, perhaps a maintainer of
any supported architecture having pre/post-modify memory access can
take a look at this?

Basically, it seems to me that if a BB has a sequence like (using C
syntax for clarity)

r1 = r2 + 42;
r3 = *r1++;
r4 = *(r2 + 42);

--then the cse pass overlooks the modification of r1 by the second
instruction, and changes the last instruction to "r4 = *r1"