[MLIR] Add llvm.mlir.cast op for semantic preserving cast between dialect types.

Summary: See discussion here: https://llvm.discourse.group/t/rfc-dialect-type-cast-op/538/11

Reviewers: ftynse

Subscribers: bixia, sanjoy.google, mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, aartbik, liufengdb, Joonsoo, llvm-commits

Differential Revision: https://reviews.llvm.org/D75141