[Canonicalizer] Process regions top-down instead of bottom up & reuse existing constants.
Original commit message:
- Change the canonicalizer to walk the function in top-down order instead of bottom-up order. This composes well with the "top down" nature of constant folding and simplification, reducing iterations and re-evaluation of ops in simple cases.
- Explicitly enter existing constants into the OperationFolder table before canonicalizing. Previously we would "constant fold" them and rematerialize them, wastefully recreating a bunch fo constants, which lead to pointless memory traffic.
Both changes together provide a 33% speedup for canonicalize on some mid-size
One artifact of this change is that the constants generated in normal pattern
application get inserted at the top of the function as the patterns are applied.
Because of this, we get "inverted" constants more often, which is an aethetic
change to the IR but does permute some testcases.
Differential Revision: https://reviews.llvm.org/D99006