Fix SRA issue with -ftrivial-auto-var-init= [PR121894]

Fix SRA issue with -ftrivial-auto-var-init= [PR121894]

In tree-sra.cc, for the following stmt (initialization in source code):
s = {};

for the above lhs "s", the field "grp_assignment_write" of the created
struct access is 1;

however, for the following stmt (compiler added initialization):
s = .DEFERRED_INIT (size, init_type, &"s"[0]);

for the above lhs "s", the field "grp_assignment_write" of the created
struct access is 0;

Since the field "grp_assignment_write" of the struct access for the
corresponding LHS "s" is not set correctly when the RHS is .DEFERRED_INIT,
SRA phase didn't do a correct transformation for call to .DEFERRED_INIT.

To fix this issue, we should set the field "grp_assignment_write" correctly
for .DEFERRED_INIT.

	PR tree-optimization/121894

gcc/ChangeLog:

	* tree-sra.cc (scan_function): Set grp_assignment_write to 1 when
	specially handle call to .DEFERRED_INIT.

gcc/testsuite/ChangeLog:

	* g++.dg/opt/auto-init-sra-pr121894.C: New test.