LLVM dialect: prefix auxiliary operations with "mlir."

Some of the operations in the LLVM dialect are required to model the LLVM IR in
MLIR, for example "constant" operations are needed to declare a constant value
since MLIR, unlike LLVM, does not support immediate values as operands.  To
avoid confusion with actual LLVM operations, we prefix such axuiliary
operations with "mlir.".

PiperOrigin-RevId: 266942838