[IR/Verifier] Don't visit DISubprograms more than needed.

Before this patch we happened to visit twice, one when scanning
MDNodes and the other one while visiting the function. Remove
the explicit call to visitDISubprogram there, so we don't emit
the same error twice in case the verifier fail and we save some
time when running it.
Thanks to Justin Bogner for the report and Adrian for the quick
review!

PR: 31995
llvm-svn: 295537