[TableGen] Better support for predicate and rewrite rule specification

[TableGen] Better support for predicate and rewrite rule specification

    Currently predicates are written with positional placeholders `{N}` and rely on
    `formatv` as the engine to do substitution. The problem with this approach is that
    the definitions of those positional placeholders are not consistent; they are
    entirely up to the defining predicate of question. For example, `{0}` in various
    attribute constraints is used to mean the attribute, while it is used to main the
    builder for certain attribute transformations. This can become very confusing.

    This CL introduces `tgfmt` as a new mechanism to better support for predicate and
    rewrite rule specification. Instead of entirely relying on positional placeholders,
    `tgfmt` support both positional and special placeholders. The former is used for
    DAG operands. The latter, including $_builder, $_op, $_self, are used as special
    "hooks" to entities in the context. With this, the predicate and rewrite rules
    specification can be more consistent is more readable.

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PiperOrigin-RevId: 243249671