[mlir][SCF][bufferize][NFC] Implement resolveConflicts for ParallelInsertSliceOp

This was previous implemented as part of the BufferizableOpInterface of ForEachThreadOp. Moving the implementation to ParallelInsertSliceOp to be consistent with the remaining ops and to have a nice example op that can serve as a blueprint for other ops.

Differential Revision: https://reviews.llvm.org/D128666