[mlir][Transforms][NFC] Move `ReconcileUnrealizedCasts` implementation (#104671)

Move the implementation of `ReconcileUnrealizedCasts` to
`DialectConversion.cpp`, so that it can be called from there in a future
commit.

This commit is in preparation of decoupling argument/source/target
materializations from the dialect conversion framework. The existing
logic around unresolved materializations that predicts IR changes to
decide if a cast op can be folded/erased will become obsolete, as
`ReconcileUnrealizedCasts` will perform these kind of foldings on fully
materialized IR.

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Co-authored-by: Markus Böck <markus.boeck02@gmail.com>

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