[mlir][Transforms] `GreedyPatternRewriteDriver`: Hash ops separately (#78312)

The greedy pattern rewrite driver has multiple "expensive checks" to
detect invalid rewrite pattern API usage. As part of these checks, it
computes fingerprints for every op that is in scope, and compares the
fingerprints before and after an attempted pattern application.

Until now, each computed fingerprint took into account all nested
operations. That is quite expensive because it walks the entire IR
subtree. It is also redundant in the expensive checks because we already
compute a fingerprint for every op.

This commit significantly improves the running time of the "expensive
checks" in the greedy pattern rewrite driver.