[ArgPromotion] Protect harder against recursive promotion (PR42028)

In addition to the self-recursion check, also check whether there
is more than one node in the SCC, which implies that there is a
larger cycle. I believe checking SCC structure (rather than
something like norecurse) is the right thing to do here, because
this is specifically about preventing infinite loops over the SCC.

Fixes https://github.com/llvm/llvm-project/issues/42028.

Differential Revision: https://reviews.llvm.org/D119418