[X86] atomic-load-store.ll - cleanup test check-prefix hierarchies to improve reuse and fix missing AVX2/AVX512 checks (llvm#165552)

-mcpu=x86-64 is still SSE codegen, and there were missing AVX2/AVX512 checks where the common CHECK-AVX prefix clashed

Noticed while reviewing llvm#148897

Author: RKSimon