[mlir] Flip dialects to _Prefixed

Following
https://llvm.discourse.group/t/psa-ods-generated-accessors-will-change-to-have-a-get-prefix-update-you-apis/4476
these have been flipped to both for ~4 weeks, flipping to _Prefixed.

Differential Revision: https://reviews.llvm.org/D115585