[AVX-512] Make VEX encoded FMA instructions available when AVX512 is enabled...

[AVX-512] Make VEX encoded FMA instructions available when AVX512 is enabled regardless of whether +fma was added on the command line.

We weren't able to handle isel of the 128/256-bit FMA instructions when AVX512F was enabled but VLX and FMA weren't.

I didn't mask FeatureAVX512 imply FeatureFMA as I wasn't sure I wanted disabling FMA to also disable AVX512. Instead we just can't prevent FMA instructions if AVX512 is enabled.

Another option would be to promote 128/256-bit to 512-bit, do the operation and extract it. But that requires a lot of extra isel patterns. Since no CPUs exist that support AVX512, but not FMA just using the VEX instructions seems better.

llvm-svn: 298051