This commit improves the `allowPatternRollback` flag handling in the
dialect conversion driver. Previously, this flag was used to merely
detect cases that are incompatible with the new One-Shot Dialect
Conversion driver. This commit implements the driver itself: when the
flag is set to "false", all IR changes are materialized immediately,
bypassing the `IRRewrite` and `ConversionValueMapping` infrastructure.
A few selected test cases now run with both the old and the new driver.
RFC:
https://discourse.llvm.org/t/rfc-a-new-one-shot-dialect-conversion-driver/79083
7e7c9d975e