testsuite/102517 - fix FAIL of gcc.dg/pr78408-1.c with OImode availability

testsuite/102517 - fix FAIL of gcc.dg/pr78408-1.c with OImode availability

This fixes the testcase which looks for variants of memcpy after
memset folding which is disturbed when we expand the memcpy inline
earlier which in fact performs the desired optimization but makes
the dump file not match.  For the ease of testing the following
adjusts the smaller structure size to be no longer power-of-two
which avoids the inline expansion.

2021-09-29  Richard Biener  <rguenther@suse.de>

	PR testsuite/102517
	* gcc.dg/pr78408-1.c: Make S not power-of-two size.