[mlir] Replace AbstractOperation::classof with a ClassID instance.

Summary: This field is currently not used by anything, and using a ClassID instance provides better support for more efficient classof.

Reviewers: mehdi_amini, nicolasvasilache

Reviewed By: mehdi_amini

Subscribers: merge_guards_bot, jpienaar, burmako, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, liufengdb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72822