[Bug rtl-optimization/66552] Missed optimization when shift amount is result of signed modulus

[segher at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66552

--- Comment #12 from Segher Boessenkool <segher at gcc dot gnu.org> ---
Sure, but what makes shift amount special here?  This works just fine in
any other expression as well.  That is, for unsigned n; for negative numbers
modulo works differently: it returns 0 or negative, so it is either UB, or
the masking works as expected.  Is there not a better way to express that?