[SDAG] avoid generating libcall to function with same name

This is a potentially better alternative to D131452 that also
should avoid the infinite loop bug from:
issue #56403

This is again a minimal fix to reduce merging pain for the
release. But if this makes sense, then we might want to guard
all of the RTLIB generation (and other libcalls?) with a
similar name check.

Differential Revision: https://reviews.llvm.org/D131521