Improved diagnostics for SSAValue.erase()

[MichalPitr]

SSAValue.erase() raises a generic exception "Attempting to delete SSA value that still has uses."

It would be more useful if it also printed out the specific operation.

This can be achieved by:

   def erase(self, safe_erase: bool = True) -> None:
        """
        Erase the value.
        If safe_erase is True, then check that no operations use the value anymore.
        If safe_erase is False, then replace its uses by an ErasedSSAValue.
        """
        if safe_erase and len(self.uses) != 0:
            raise Exception(
                f"Attempting to delete SSA value that still has uses: {self}")
        self.replace_by(ErasedSSAValue(self.typ))