[mlir][DialectConversion] Do not prematurely drop unused cast operations

The rewrite logic has an optimization to drop a cast operation after
rewriting block arguments if the cast operation has no users. This is
unsafe as there might be a pending rewrite that replaced the cast operation
itself and hence would trigger a second free.

Instead, do not remove the casts and leave it up to a later canonicalization
to do so.

Differential Revision: https://reviews.llvm.org/D92184