ipa: Cgraph verification fix (PR 94856)

ipa: Cgraph verification fix (PR 94856)

PR 94856 is a call graph verifier error.  We have a method which (in
the course of IPA-CP) loses its this pointer because it is unused and
the pass then does not clone all the this adjusting thunks and just
makes the calls go straight to the new clone - and then the verifier
complains that the edge does not seem to point to a clone of what it
used to.  This looked weird because the verifier actually has logic
detecting this case but it turns out that it is confused by inliner
body-saving mechanism which invents a new decl for the base function.

Making the inlining body-saving mechanism to correctly set
former_clone_of allows us to detect this case too.  Then we pass this
particular round of verification but the subsequent one fails because
we have inlined the function into its former thunk - which
subsequently does not have any callees, but the verifier still access
them and segfaults.  Therefore the patch also adds a test whether the
a former hunk even has any call.

2020-04-30  Martin Jambor  <mjambor@suse.cz>

	PR ipa/94856
	* cgraph.c (clone_of_p): Also consider thunks whih had their bodies
	saved by the inliner and thunks which had their call inlined.
	* ipa-inline-transform.c (save_inline_function_body): Fill in
	former_clone_of of new body holders.

	PR ipa/94856
	* g++.dg/ipa/pr94856.C: New test.