[mlir] Add `enableSplitting` and `insertMarkerInOutput` options to `splitAndProcessBuffer`

`enableSplitting` simply enables/disables whether we should split
or use the full buffer. `insertMarkerInOutput` toggles if split markers
should be inserted in between prcessed output chunks.

These options allow for merging the duplicate code paths we have
when splitting is optional.

Differential Revision: https://reviews.llvm.org/D128764