Fix another verifier crash where a GC intrinsic would look at the internals of...

Fix another verifier crash where a GC intrinsic would look at the internals of another intrinsic in order to verify itself.

This causes a crash if the referenced intrinsic was malformed.  In this case, we
would already have reported an error on the referenced intrinsic, but then
crashed on the second one when it tried to introspect the first without
error checking.

llvm-svn: 231910