[X86] Add an avx512bw command line to the avx512-vec-cmp.ll test. Add some additional test cases.

Additional test cases cover selects with i16/i8 conditions that are only 128/256-bits wide, but the compares are 512-bits wide and can only produce k-registers. We should be able to artificially widen the selects to avoid moving the k-register to an xmm/ymm register.

llvm-svn: 322449