feat: convert x.sdiv (2^k) into x.sshiftRight k

[bollu]

Subtleties abound, mostly to do with intMin, and when 2^k is a sensible number, when it is intMin, and when it overflows.

[bollu]

The relevant section from Hacker's delight: 10-1

[bollu]

This is pointless, because sdiv is already unfolded into udiv, which knows how to handle power-of-two. That encoding is competitive with this encoding. The only thing that could make this faster is to use a branchless encoding, but there's higher priority stuff right now, so I won't pursue this. Closing.