testsuite: XFAIL SLP induction tests for VL vectors

testsuite: XFAIL SLP induction tests for VL vectors

We don't yet support SLP inductions for variable-length vectors,
so this patch XFAILs some associated tests.

(Inductions aren't inherently difficult to support.  It just hasn't
been done yet.)

gcc/testsuite/
	* gcc.dg/vect/pr97678.c: XFAIL test for SLP vectorization
	for variable-length vectors.
	* gcc.dg/vect/pr97835.c: Likewise.
	* gcc.dg/vect/slp-49.c: Likewise.
	* gcc.dg/vect/vect-outer-slp-1.c: Likewise.
	* gcc.dg/vect/vect-outer-slp-2.c: Likewise.
	* gcc.dg/vect/vect-outer-slp-3.c: Likewise.