Optimize bitmask immediates

[kddnewton]

No description provided.

[XrXr]

Sure! It's public domain code, but we might want to mention https://dougallj.wordpress.com/2021/10/30/bit-twiddling-optimising-aarch64-logical-immediate-encoding-and-decoding/ in the code though.

Also we should maybe test this more with a lookup table and such? Even though dougallj's version has have more eyes on it somethings might've gotten lost in translation.

[kddnewton]

Ehh he pretty explicitly says no need to mention. I suppose I can put a comment in though.

We could add a lookup table, but I feel pretty confident with the existing test suite. I'll poke around a bit more though just to be super safe. I'll see if I can run it through https://gist.github.com/dinfuehr/51a01ac58c0b23e4de9aac313ed6a06a.

[XrXr]

Also the full version has some comments which we could copy over: https://gist.github.com/dougallj/97d8621d4542ba31e004acc8075fac14

Another testing idea for #[cfg(feature = "disasm")]: roll a random u64 and encode it if possible. Check the result against the disassembly.

[kddnewton]

Okay I just tested every possible value.

test result: ok. 5354 passed; 0 failed; 0 ignored; 0 measured; 276 filtered out; finished in 0.27s