testsuite: Adjust possibly fragile slp-perm-9.c [PR104015]

testsuite: Adjust possibly fragile slp-perm-9.c [PR104015]

As Richard pointed out in PR104015, the test case slp-perm-9.c
can be fragile when vectorizer tries to use different
vectorisation strategies.

As suggested, this patch tries to make the check not sensitive
on the re-trying times by removing the times checking.  To still
retain the test coverage on unnecessary re-trying, for example
it exposes this PR104015 on Power9, I added two test cases to
powerpc testsuite.

gcc/testsuite/ChangeLog:

	PR tree-optimization/104015
	* gcc.dg/vect/slp-perm-9.c: Adjust.
	* gcc.target/powerpc/pr104015-1.c: New test.
	* gcc.target/powerpc/pr104015-2.c: New test.