[Bug target/66511] [avr] whole-byte shifts not optimized away for uint64_t

[matthijs at stdin dot nl]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66511

--- Comment #2 from Matthijs Kooijman <matthijs at stdin dot nl> ---
So, IIUC, this is quite hard to fix? Either you use lib functions, which
prevents the optimizer from just relabeling or coyping registers to apply
shifting, or you don't and then more complex operations will become very
verbose and messy?

Would it make sense (and be possible) to add a special case to not use lib
functions for shifts by a constant number of bits that is also a multiple of 8?
At first glance, that would make a lot of common cases (where an integer is
decomposed into separate bytes or other parts) a lot faster, while still
keeping the lib functions for more complex operations?