[mlir][Transforms] `GreedyPatternRewriteDriver`: Better expensive checks encapsulation (#78175)

This change moves most IR verification logic (which is part of the
expensive checks) into `DebugFingerPrints` and renames the struct to
`ExpensiveChecks`. This isolates the debugging logic better from the
remaining code.

This commit also removes a redundant check: the IR is no longer verified
after a failed pattern application. We already assert that the IR did
not change. (We know that the IR was valid before the attempted pattern
application.)