sra: Fix bug in grp_write propagation (PR 97009)

sra: Fix bug in grp_write propagation (PR 97009)

SRA represents parts of aggregates which are arrays accessed with
unknown index as "unscalarizable regions."  When there are two such
regions one within another and the outer is only read whereas the
inner is written to, SRA fails to propagate that write information
across assignments.  This means that a second aggregate can contain
data while SRA thinks it does not and the pass can wrongly eliminate
big chunks of assignment from that second aggregate into a third
aggregate, which is what happens in PR 97009.

Fixed by checking all children of unscalariable accesses for the
grp_write flag.

gcc/ChangeLog:

2021-03-31  Martin Jambor  <mjambor@suse.cz>

	PR tree-optimization/97009
	* tree-sra.c (access_or_its_child_written): New function.
	(propagate_subaccesses_from_rhs): Use it instead of a simple grp_write
	test.

gcc/testsuite/ChangeLog:

2021-03-31  Martin Jambor  <mjambor@suse.cz>

	PR tree-optimization/97009
	* gcc.dg/tree-ssa/pr97009.c: New test.