testsuite: enable vector MIN for vec-narrow-1.C on x86

The positive test only folds when the wide and narrow vector MIN operations
are available.  Default x86_64 lowers the V2DI MIN before the narrowing rule
can see it, so the dump still contains the wide type.

Enable AVX512VL and AVX512DQ for x86 targets to enable the type.

Tested on x86_64-pc-linux-gnu.

gcc/testsuite/ChangeLog:

	* g++.dg/tree-ssa/vec-narrow-1.C: Enable AVX512VL and AVX512DQ on
	x86 targets.

Signed-off-by: Kyrylo Tkachov <ktkachov@nvidia.com>