[spirv] Start the SPIR-V dialect

    This CL sets up the basic structure for a SPIR-V dialect: operation
    definition specification, dialect registration, testing, etc.
    A single op, FMul, is defined and tested to showcase.

    The SPIR-V dialect aims to be a simple proxy for the SPIR-V binary format
    to enable straightforward and lightweight conversion from/to the binary
    format. Ops in this dialect should stay as the same semantic level and
    try to be a mechanical mapping to the corresponding SPIR-V instructions;
    but they can deviate representationally to allow using MLIR mechanisms.

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PiperOrigin-RevId: 250040830