[X86][AVX512] Refine some avx512er intrinsics tests. NFC.

The modified tests should test the masked intrinsics.
Currently the mask is constant, which with a future patch (https://reviews.llvm.org/D32805) will cause the intrinsics to be replaced with an unmasked version.
This patch changes the constant mask to be a variable one.

llvm-svn: 302529