Fix PRFCHW test on non-x86 builds

- 'prefetch' intrinsics are only lowered when SSE is available. On non-X86
  builds, 'generic' CPU is used and stops lowering any prefetch intrinsics.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178046 91177308-0d34-0410-b5e6-96231b3b80d8