Static identifier checking of Yul.
This is a very simple check that all the identifiers in Yul code are well-formed. This check may go away altogether if we were to build these constraints into the definition of identifiers, which we may do at some point. But for now, it is an explicit check.
We only check identifiers at the points in which they are declared. The static safey checks ensure that every used identifier is declared, so there is no need to check uses of identifiers.