Refine implementation of HLO_SpeculatableIfAllInputsStatic

[mlevesquedion]

The logic is a little bit too conservative currently:

func.func @xor(%dynamic_arg: tensor<?xi64>) {
  %speculatable = stablehlo.xor %dynamic_arg, %dynamic_arg : (tensor<?xi64>, tensor<?xi64>) -> tensor<?xi64>
}

Currently, we consider the above not speculatable, but actually because the operands are the same there cannot be a mismatch at runtime so this is speculatable.