Fix handling of thunks in ipa-modref

Fix handling of thunks in ipa-modref

Thunks are not transparent for ipa-modref summary since it cares about offsets
from pointer parameters and also for virtual thunk about the read from memory
in there.  We however use function_or_virtual_thunk_symbol to get the summary
that may lead to wrong code (and does in two testsuite testcases with patch
I am working on).  This is a first aid fix that is bacportable to gcc 11.
We could easily produce summary for thunk on demand.  I will look into it
incrementally.  It is not very important since we usually inline the thunk when
we devirutalize...

Bootstrapped/regtested x86_64-linux, will commit it shortly.

gcc/ChangeLog:

2021-12-11  Jan Hubicka  <hubicka@ucw.cz>

	* ipa-modref.c (get_modref_function_summary): Use ultimate_alias_target.
	(ignore_edge): Likewise.
	(compute_parm_map): Likewise.
	(modref_propagate_in_scc): Likewise.
	(modref_propagate_flags_in_scc): Likewise.