[SDAG] try harder to remove a rotate from X == 0

https://alive2.llvm.org/ce/z/mJP7XP

This can be viewed as expanding the compare into and/or-of-compares:
https://alive2.llvm.org/ce/z/bkZYWE
followed by reduction of each compare.

This could be extended in several ways:
1. There's a (X & Y) == -1 sibling.
2. We can recurse through more than 1 'or'.
3. The fold could be generalized beyond rotates - any operation that
   only changes the order of bits (bswap, bitreverse).

This is a transform noted in D111530.