testsuite: Add testcase coverage for already fixed [PR96671]

The fix for this PR didn't come with any test coverage, I've added
tests that make sure we optimize it no matter what order of the x ^ y ^ z
operands is used.

2021-01-15  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/96671
	* gcc.dg/tree-ssa/pr96671-1.c: New test.
	* gcc.dg/tree-ssa/pr96671-2.c: New test.