[mlir][arith] Add arith.constant materialization helper

This adds `arith::ConstantOp::materialize`, which builds a constant from
an attribute and type only if it would result in a valid op. This is
useful for dialect `materializeConstant` hooks, and allows for removing
the previous `Attribute, Type` builder which was only used during
materialization.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D148491