Add 'dot' opcode

[seanmcl]

This was missing from the spec, but is parsed by the C++ code.

https://github.com/openxla/stablehlo/blob/46d146850e1b5ae701f493c2d92875c757b85a7c/stablehlo/dialect/StablehloOps.cpp#L557

Tested with

"builtin.module"() ({
  "func.func"() <{function_type = (tensor<28x28xf32>, tensor<784x10xf32>, tensor<1x10xf32>) -> tensor<1x10xf32>, sym_name = "main"}> ({
  ^bb0(%arg0: tensor<28x28xf32>, %arg1: tensor<784x10xf32>, %arg2: tensor<1x10xf32>):
    %0 = "stablehlo.reshape"(%arg0) : (tensor<28x28xf32>) -> tensor<1x784xf32>
    %1 = "stablehlo.dot"(%0, %arg1) : (tensor<1x784xf32>, tensor<784x10xf32>) -> tensor<1x10xf32>
    %2 = "stablehlo.add"(%1, %arg2) : (tensor<1x10xf32>, tensor<1x10xf32>) -> tensor<1x10xf32>
    %3 = "stablehlo.constant"() <{value = dense<0.000000e+00> : tensor<1x10xf32>}> : () -> tensor<1x10xf32>
    %4 = "stablehlo.maximum"(%2, %3) : (tensor<1x10xf32>, tensor<1x10xf32>) -> tensor<1x10xf32>
    "func.return"(%4) : (tensor<1x10xf32>) -> ()
  }) : () -> ()
}) : () -> ()

Will add this as a unit test once I figure out how to do lean unit testing.