[mlir] Fix crash in RewriterGen when a `TypeConstraint` is not given an argument

The code assumes that a TypeConstraint in the additional constraints list specifies precisely one argument.
If the user were to not specify any, it'd result in a crash. If given more than one, the additional ones were ignored.

This patch fixes the crash and disallows user errors by adding a check that a single argument is supplied to the TypeConstraint

Differential Revision: https://reviews.llvm.org/D118763