[X86] Add a AVX512VBMI command line to min-legal-vector-width.ll. Always enable fast-variable-shuffle

Trying to minimize the features we need to manipulate when this
is updated for D67259.

The VBMI is interesting because it enables some improved combining
for truncates.

I enabled fast-variable-shuffle because all the CPUs we're going
to add implicitly enable it. So they can share check lines.

llvm-svn: 371261

Author: topperc