[mlir][Transforms] Dialect conversion: Move `hasRewrite` to expensive checks (#119848)

The dialect conversion has various checks that detect incorrect API
usage in patterns. One of these checks turned out to be quite expensive
(N*M complexity where N is the number of block rewrites and M is the
total number of rewrites) in NVIDIA-internal workloads: Checking that a
block is not converted multiple times.

This check iterates over the stack of all rewrites, which can be large.
We saw `hasRewrite` being called around 45000 times with an average
rewrite stack size of 500000.

This PR moves the check to `MLIR_ENABLE_EXPENSIVE_PATTERN_API_CHECKS`.
For consistency reasons, the other `hasRewrite`-based check is also
moved there.

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