[mlir] Add a new `ConstantLike` trait to better identify operations that represent a "constant".

The current mechanism for identifying is a bit hacky and extremely adhoc, i.e. we explicit check 1-result, 0-operand, no side-effect, and always foldable and then assume that this is a constant. Adding a trait adds structure to this, and makes checking for a constant much more efficient as we can guarantee that all of these things have already been verified.

Differential Revision: https://reviews.llvm.org/D76020