ipa/96291: don't crash on unoptimized lto functions

In PR ipa/96291 the test contained an SCC with one
unoptimized function. This tricked ipa-cp into NULL dereference.

has_undead_caller_from_outside_scc_p() did not take into account
that unoptimized funtions don't have IPA summary analysis. And
dereferenced NULL pointer causing an ICE.

gcc/
	PR ipa/96291
	* ipa-cp.c (has_undead_caller_from_outside_scc_p): Consider
	unoptimized callers as undead.

gcc/testsuite/
	PR ipa/96291
	* gcc.dg/lto/pr96291_0.c: New testcase.
	* gcc.dg/lto/pr96291_1.c: Support file.
	* gcc.dg/lto/pr96291_2.c: Likewise.
	* gcc.dg/lto/pr96291.h: Likewise.