[Bug rtl-optimization/84660] Combine doing wrong optimization for 64 bits with SHIFT_COUNT_TRUNCATED target

[wilson at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84660

--- Comment #3 from Jim Wilson <wilson at gcc dot gnu.org> ---
I screwed up the logic and need to redo the patch.  The basic idea is sound
though.