ConvertStandardToLLVM: replace assertion with graceful failure

The assertion was introduced in the early days of dialect conversion
infrastructure when we had the matching function separate from the rewriting
function. The infrastructure evolved to have a common matchAndRewrite function
and the separate matching function was dropped without chaning the rewriting
that became matchAndRewrite. This has led to assertion being triggered. Return
a matchFailure instead of failing an assertion on unsupported types.

Closes tensorflow/mlir#230

PiperOrigin-RevId: 281113741