[SDAG] add check to sext-of-setcc fold to bypass changing a legal op

I accidentaly pushed a draft of D103280 that was discussed
during the review, but it was not supposed to be the final
version.

Rather than revert and recommit, I'm updating the existing
code. This way we have a record of the codegen diff that
would result if we decide to remove this predicate in the
future.