[MLIR][Linalg] Named op 'add' element-wise

This adds the first strict element-wise named op to Linalg.

The semantics here is to not allow auto-cast, broadcast semantics and to
restrict the operations only to identical types. The remaining semantics
must come in the form of surrounding operations on operands, to avoid
ambiguity.

Examples:
```
  // Cast int-to-fp
  %0 = linalg.copy ins(%in: tensor<32x32xi32>)
                   outs(%out: tensor<32x32xf32>)
  %1 = linalg.add  ins(%arg, %0: tensor<32x32xf32>, tensor<32x32xf32>)
                   outs(%0: tensor<32x32xf32>)

  // This can be lowered to
  %1 = linalg.generic {...}
            ins(%arg, %in: tensor<32x32xf32>, tensor<32x32xi32>)
            outs(%0: tensor<32x32xf32>) {
    ^bb0(%a: f32, %i: i32, %out: f32):
      %f = arith.uitofp %i : f32
      %0 = arith.addf %a, %f : f32
      linalg.yield %0 : f32
  }

  // Broadcast
  %0 = linalg.broadcast ins(%in: tensor<32xf32>)
                        init(%out: tensor<32x32xf32>)
  %1 = linalg.add  ins(%arg, %0: tensor<32x32xf32>, tensor<32x32xf32>)
                   outs(%0: tensor<32x32xf32>)

  // This can be lowered to
  #bcast_map = affine_map<(d0, d1) -> (d0)>
  %1 = linalg.generic {... #bcast_map] }
            ins(%arg, %in: tensor<32x32xf32>, tensor<32xf32>)
            outs(%0: tensor<32x32xf32>) {
    ^bb0(%a: f32, %b: f32, %out: f32):
      %0 = arith.addf %a, %b : f32
      linalg.yield %0 : f32
  }
```

Once this gets accepted, other arithmetic and maths operations will be
added accordingly, with the same semantics.

Differential Revision: https://reviews.llvm.org/D154500