[X86] Turn v16i16->v16i8 truncate+store into a any_extend+truncstore if we...

[X86] Turn v16i16->v16i8 truncate+store into a any_extend+truncstore if we avx512f, but not avx512bw.

Ideally we'd be able to represent this truncate as a any_extend to
v16i32 and a truncate, but SelectionDAG doens't know how to not
fold those together.

We have isel patterns to use a vpmovzxwd+vpdmovdb for the truncate,
but we aren't able to simultaneously fold the load and the store
from the isel pattern. By pulling the truncate into the store we
can successfully hide it from the DAG combiner. Then we can isel
pattern match the truncstore and load+any_extend separately.

llvm-svn: 364163