testsuite: xfail shrink-wrapping-vector-1.C until PR125400 is fixed

After r17-603-gacfdad706d8acc, shrink-wrapping-vector-1.C started to
fail because of a missed optimization. So let's mark it as xfail
for the C++ versions where it fails (that is c++17 and down).

Pushed as obvious after testing to see if the testcase xfails correctly.

gcc/testsuite/ChangeLog:

	* g++.dg/opt/shrink-wrapping-vector-1.C: xfail for C++17down.

Signed-off-by: Andrew Pinski <andrew.pinski@oss.qualcomm.com>