[mlir] Symbol DCE ignores unknown symbols

Instead of failing when it encounters a reference to an unknown symbol, Symbol DCE should ignore them. References to unknown symbols do not affect the overall function of Symbol DCE, so it should not need to fail when it encounters one.

In general, requiring that symbol references always be valid rather than only when necessary can be overly conservative.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D116047