The logic in EarlyCSE that looks through 'not' operations in the
predicate recognizes e.g. that select (not (cmp sgt X, Y)), X, Y is
equivalent to select (cmp sgt X, Y), Y, X. Without this change,
however, only the latter is recognized as a form of smin X, Y, so the
two expressions receive different hash codes. This leads to missed
optimization opportunities when the quadratic probing for the two hashes
doesn't happen to collide, and assertion failures when probing doesn't
collide on insertion but does collide on a subsequent table grow
This change inverts the order of some of the pattern matching, checking
first for the optional not and then for the min/max/abs patterns, so
that e.g. both expressions above are recognized as a form of smin X, Y.
It also adds an assertion to isEqual verifying that it implies equal
hash codes; this fires when there's a collision during insertion, not
just grow, and so will make it easier to notice if these functions fall
out of sync again.