Improve handling of return slot in ipa-pure-const and modref.

Improve handling of return slot in ipa-pure-const and modref.

while preparing testcase for return slot tracking I noticed that both
ipa-pure-const and modref treat return slot writes as non-local which prevents
detecting functions as pure or not modifying global state.  Fixed by making
points_to_local_or_readonly_memory_p to special case return slot.  This is bit
of a side case, but presently at all uses of
points_to_local_or_readonly_memory_p we want to handle return slot this way.

I also noticed that we handle gimple copy unnecesarily pesimistically.  This
does not make difference right now since we do no not track non-scalars, but
I fixed it anyway.

Bootstrapped/regtested x86_64-linux, comitted.

gcc/ChangeLog:

	* ipa-fnsummary.c: Include tree-dfa.h.
	(points_to_local_or_readonly_memory_p): Return true on return
	slot writes.
	* ipa-modref.c (analyze_ssa_name_flags): Fix handling of copy
	statement.

gcc/testsuite/ChangeLog:

	* g++.dg/ipa/modref-1.C: New test.