ipa-cp: Fix assert triggering with -fno-toplevel-reorder (PR 106260)

ipa-cp: Fix assert triggering with -fno-toplevel-reorder (PR 106260)

with -fno-toplevel-reorder (and -fwhole-program), there apparently can
be local functions without any callers.  This is something that IPA-CP
does not like because its propagation verifier checks that local
functions do not end up with TOP in their lattices.  Therefore there
is an assert checking that all call-less unreachable functions have
been removed, which tigers in PR 106260 with these two options.

This patch detects the situation and marks the lattices as variable,
thus avoiding both the assert trigger and the verification failure.

gcc/ChangeLog:

2022-07-13  Martin Jambor  <mjambor@suse.cz>

	PR ipa/106260
	* ipa-cp.cc (initialize_node_lattices): Replace assert that there are
	callers with handling that situation when -fno-toplevel_reorder.

gcc/testsuite/ChangeLog:

2022-07-13  Martin Jambor  <mjambor@suse.cz>

	PR ipa/106260
	* g++.dg/ipa/pr106260.C: New test.