Revert "[Canonicalizer] Process regions top-down instead of bottom up & reuse existing constants."

This reverts commit 361b7d12 by Chris
Lattner <clattner@nondot.org> dated Fri Mar 19 21:22:15 2021 -0700.

The change to the greedy rewriter driver picking a different order was
made without adequate analysis of the trade-offs and experimentation. A
change like this has far reaching consequences on transformation
pipelines, and a major impact upstream and downstream. For eg., one
can’t be sure that it doesn’t slow down a large number of cases by small
amounts or create other issues. More discussion here:
https://llvm.discourse.group/t/speeding-up-canonicalize/3015/25

Reverting this so that improvements to the traversal order can be made
on a clean slate, in bigger steps, and higher bar.

Differential Revision: https://reviews.llvm.org/D99329