[mlir][arith] Allow to specify `constFoldBinaryOp` result type

This enables us to use the common fold helpers on elementwise ops that
produce different result type than operand types, e.g., `arith.cmpi` or
`arith.addui_extended`.

Use the updated helper to teach `arith.cmpi` to fold constant vectors.

Reviewed By: Mogball

Differential Revision: https://reviews.llvm.org/D143779