[mlir][LLVMIR] Support memory model relaxation annotations (MMRA) (#157770) This commit adds support for exportind and importing MMRA data in the LLVM dialect. MMRA is a potentilly-discardable piece of metadata that can be placed on any operation that touches memory (fences, loads, stores, atomics, and intrinsics that operate on memory). It includes one (technically zero) ome more prefix:suffix string pairs which indicate ways in which the LLVM memory model can be relaxed for these annotations. At the MLIR level, each tag is represented with a `#llvm.mmra_tag<"prefix":"suffex">` attribute, and the MMRA metadata as a whole is represented as a discardable llvm.mmra attribute. (This discardability both allows us to transparently enable MMRA for wrapper dialects like ROCDL and ensures that MLIR passes which don't know about MMRA combining will, conservatively, discard the annotations, per the LLVM spec). --------- Co-authored-by: Tobias Gysi <tobias.gysi@nextsilicon.com>
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