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When a conversion pattern is initialized without a type converter, the driver implementation currently looks up the most recently mapped value. This is undesirable because the most recently mapped value could be a materialization. I.e., the type of the value being looked up could depend on which other patterns have run before. Such an implementation makes the type conversion infrastructure fragile and unpredictable. The current implementation also contradicts the documentation in the markdown file. According to that documentation, the values provided by the adaptor should match the types of the operands of the match operation when running without a type converter. This mechanism is not desirable, either, for two reasons: 1. Some patterns have started to rely on receiving the most recently mapped value. Changing the behavior to the documented behavior will cause regressions. (And there would be no easy way to fix those without forcing the use of a type converter or extending the `getRemappedValue` API.) 2. It is more useful to receive the most recently mapped value. A value of the original operand type can be retrieved by using the operand of the matched operation. The adaptor is not needed at all in that case. To implement the new behavior, materializations are now annotated with a marker attribute. The marker is needed because not all `unrealized_conversion_cast` ops are materializations that act as "pure type conversions". E.g., when erasing an operation, its results are mapped to newly-created "out-of-thin-air values", which are materializations (with no input) that should be treated like regular replacement values during a lookup. This marker-based lookup strategy is also compatible with the One-Shot Dialect Conversion implementation strategy, which does not utilize the mapping infrastructure anymore and queries all necessary information by examining the IR.
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