testsuite: Fix fold-vecperm-1.c for targets that don't have vectors

testsuite: Fix fold-vecperm-1.c for targets that don't have vectors

This testcase is testing at optimization but with targets that don't
have vectors it will fail because there will be zero VEC_PERM_EXPR.
So instead let's check earlier in say forwprop3.

Pushed as obvious after a test on x86_64-linux-gnu with -mno-sse.

gcc/testsuite/ChangeLog:

	* gcc.dg/fold-vecperm-1.c: Test at forwprop3.

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