[Bug tree-optimization/71947] [6/7 Regression] x ^ y not folded to 0 if x == y by DOM

[law at redhat dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71947

--- Comment #11 from Jeffrey A. Law <law at redhat dot com> ---
So I don't like the pain of trying to fold at each propagation step. 
Specifically, the structure of the gimple statement can change, which
invalidates the operand cache.  And the canonicalization of operands based on
their SSA_NAME_VERSIONs means it's possible to get cycles.  While I've got
something that appears to work, I'm not at all happy with it.

So I started looking at a different approach.  Just keep a record of copy
propagated objects and if we've already propagated A for B, don't copy
propagate B for A in the same statement.  We can do this with a bitmap, but
that's a lot of overhead, even when lazily initialized.  I suspect this matters
so rarely in practice that a single entry "this was the last thing copy
propagated" is sufficient.