This continues the work started in D41096
This patch adds a new subtarget feature to indicate that there are no 512-bit vectors present in the function. When combined the prefer-avx256 feature, it will disable 512-bit vectors in the legalizer. I intend to set this subtarget feature in getSubtargetImpl when the Function has a function attribute indicating the required vector width for the function is less than 512 bits.
I looked into trying to add a 512-bit register feature that could be disabled instead like D41096 proposed, but I couldn't find a good way to make the existing command line options work. The tablegen generated subtarget feature system just doesn't allow for a wrapper/alias feature that implies other features.
I had to allow VK32 to be legal with BWI and (VLX || 512-bit vectors). VK64 is only legal with BWI and 512-bit vectors. VK64 is only needed when v64i8 is legal.
I know there are still places in the code that extend narrow vectors to 512-bit to do an operation on wider vector elements and truncate back. Those will need to be split before extending instead.
I plan to add some basic tests for this as well and I'll be adding tests as I fix the various extending lowerings mentioned above.