Change syntax of regions in the generic form of operations

    The generic form of operations currently supports optional regions to be
    located after the operation type.  As we are going to add a type to each
    region in a leading position in the region syntax, similarly to functions, it
    becomes ambiguous to have regions immediately after the operation type.  Put
    regions between operands the optional list of successors in the generic
    operation syntax and wrap them in parentheses.  The effect on the exisitng IR
    syntax is minimal since only three operations (`affine.for`, `affine.if` and
    `gpu.kernel`) currently use regions.

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PiperOrigin-RevId: 246787087