[X86] Accept hasAVX512() as well as hasFMA() when generating FMA.

[X86] Accept hasAVX512() as well as hasFMA() when generating FMA.

We don't always have FMA, for example when using 'clang -mavx512f'
without an explicit CPU.

Also check for an explicit +avx512f instead of CPUs in a couple
related tests.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240616 91177308-0d34-0410-b5e6-96231b3b80d8