[Bug rtl-optimization/109592] Failure to recognize shifts as sign/zero extension

[law at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109592

--- Comment #6 from Jeffrey A. Law <law at gcc dot gnu.org> ---
I would still rather not introduce special cases for SUBREGs if we can avoid
it.  I think the question remains whether or not patching simplify-rtx's
canonicalize_shift is sufficient to fix this problem (perhaps with the
adjustment to fwprop as well).  If they are, then they would be much preferred
over the original patch which special cased SUBREGs.