[X86] When lowering v1i1/v2i1/v4i1/v8i1 load/store with avx512f, but not avx512dq...

[X86] When lowering v1i1/v2i1/v4i1/v8i1 load/store with avx512f, but not avx512dq, use v16i1 as the intermediate mask type instead of v8i1.

We still use i8 for the load/store type. So we need to convert to/from i16 to around the mask type.

By doing this we get an i8->i16 extload which we can then pattern match to a KMOVW if the access is aligned.

llvm-svn: 350989