[mlir-tblgen] Migrate away from PointerUnion::{is,get} (NFC)#119994

kazutakahirata merged 1 commit intollvm:mainllvm/llvm-project:mainfrom

kazutakahirata:cleanup_001_PointerUnion_mlir_tblgenkazutakahirata/llvm-project:cleanup_001_PointerUnion_mlir_tblgenCopy head branch name to clipboard