[Bug tree-optimization/91965] missing simplification for (C - a) << N

[amonakov at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91965

--- Comment #1 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
On a related thought, I wonder if we can canonicalize (x << CST) to (x * CST')
where CST' is 1<<CST. This way we don't need to duplicate existing
multiplication patterns if we want to apply them to left shift.