Minor modref optimization and statistics fix

Minor modref optimization and statistics fix

this patch fixes bug in tracking memory stats and also I have noticed that while
the pass takes care to stop traking things when things are obviously out of hand
it still keeps summaries that have no useful info for loads or stores and also
many summaries are just copying const/pure attributes.  This patch thus also
adds logic to detect if summary is useful and drop it early otherwise.  This
reduces number of queries to the oracle and saves memory/lto streaming.

For cc1plus LTO build (configured with --disable-plugin
--enable-checking=release --with-build-config=lto) I now get:

Alias oracle query stats:
  refs_may_alias_p: 62488734 disambiguations, 72660949 queries
  ref_maybe_used_by_call_p: 128863 disambiguations, 63393551 queries
  call_may_clobber_ref_p: 16013 disambiguations, 21776 queries
  nonoverlapping_component_refs_p: 0 disambiguations, 37628 queries
  nonoverlapping_refs_since_match_p: 19397 disambiguations, 55370 must overlaps, 75516 queries
  aliasing_component_refs_p: 54741 disambiguations, 752198 queries
  TBAA oracle: 21632692 disambiguations 52565147 queries
               15656420 are in alias set 0
               10108172 queries asked about the same object
               124 queries asked about the same alias set
               0 access volatile
               3640460 are dependent in the DAG
               1527279 are aritificially in conflict with void *

Modref stats:
  modref use: 5712 disambiguations, 31221 queries
  modref clobber: 684316 disambiguations, 1010000 queries
  1779717 tbaa queries (1.762096 per modref query)

PTA query stats:
  pt_solution_includes: 947334 disambiguations, 13601373 queries
  pt_solutions_intersect: 1011662 disambiguations, 13139565 queries

The number of queries should change, but the number of disambiguations should
not.  However comparing with stats here
https://gcc.gnu.org/pipermail/gcc-patches/2020-September/554309.html
I see about 50% drop in clobber disambiguations. There is however same drop in
other alias oracle stats.  I suppose someting changed in meanwhile on mainline
because I was basing that on older tree.  I tried to proofread changes between
mainline and branch and they seem all quite obvious.

This is consistent with what I get on tramp3d:

Alias oracle query stats:
  refs_may_alias_p: 2051320 disambiguations, 2312132 queries
  ref_maybe_used_by_call_p: 7058 disambiguations, 2088222 queries
  call_may_clobber_ref_p: 232 disambiguations, 232 queries
  nonoverlapping_component_refs_p: 0 disambiguations, 4339 queries
  nonoverlapping_refs_since_match_p: 329 disambiguations, 10200 must overlaps, 10616 queries
  aliasing_component_refs_p: 857 disambiguations, 34639 queries
  TBAA oracle: 886768 disambiguations 1670635 queries
               131572 are in alias set 0
               461689 queries asked about the same object
               0 queries asked about the same alias set
               0 access volatile
               190291 are dependent in the DAG
               315 are aritificially in conflict with void *

Modref stats:
  modref use: 430 disambiguations, 1885 queries
  modref clobber: 9657 disambiguations, 16076 queries
  19027 tbaa queries (1.183566 per modref query)

PTA query stats:
  pt_solution_includes: 311756 disambiguations, 524179 queries
  pt_solutions_intersect: 129689 disambiguations, 415878 queries

In both cases the number of disambiguations should be same (queries are not
comparable).

Bootstrapped/regtested x86_64-linux, comitted.

gcc/ChangeLog:

2020-09-23  Jan Hubicka  <hubicka@ucw.cz>

	* ipa-modref.c (modref_summary::lto_useful_p): New member function.
	(modref_summary::useful_p): New member function.
	(analyze_function): Drop useless summaries.
	(modref_write): Skip useless summaries.
	(pass_ipa_modref::execute): Drop useless summaries.
	* ipa-modref.h (struct GTY): Declare useful_p and lto_useful_p.
	* tree-ssa-alias.c (dump_alias_stats): Fix.
	(modref_may_conflict): Fix stats.