This patch is a followup to my not-yet-reviewed patch

[PATCH v4] OpenMP: Generate SIMD clones for functions with "declare target"

https://gcc.gnu.org/pipermail/gcc-patches/2022-November/606218.html

In comments on a previous iteration of that patch, I was asked to do 
something to delete unused SIMD clones to avoid code bloat; this is it.

I've implemented something like a simple mark-and-sweep algorithm. 
Clones that are used are marked at the point where the call is generated 
in the vectorizer.  The loop that iterates over functions to apply the 
passes after IPA is modified to defer processing of unmarked clones, and 
anything left over is deleted.

OK to commit this along with the above-linked patch?

-Sandra