testsuite: Add some vect_variable_length XFAILs

testsuite: Add some vect_variable_length XFAILs

This patch adds XFAILs for some tests that fail with variable-length
vectors.

For pr96573.c I'd wondered about instead extending the regexp.
The code we generate isn't very good though, so it doesn't seem
worth matching.  (Fixing the bad code is on the todo list.)

gcc/testsuite/
	* g++.dg/tree-ssa/pr83518.C: XFAIL for vect_variable_length.
	* gcc.dg/pr96573.c: Likewise.
	* gcc.dg/tree-ssa/pr84512.c: Likewise.
	* gcc.dg/vect/bb-slp-43.c: Likewise.
	* gcc.dg/vect/slp-reduc-11.c: Likewise.