[Canonicalize] Switch the default setting to "top down".

This provides a sizable compile time improvement by seeding
the worklist in an order that leads to less iterations of the
worklist.

This patch only changes the behavior of the Canonicalize pass
itself, it does not affect other passes that use the
GreedyPatternRewrite driver

Differential Revision: https://reviews.llvm.org/D103053