[SLP][X86] Split prefer-256-bit 'AVX256BW' tests from AVX2 checks

Fixes SLP test issue with D56636

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@351199 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed