[AVX512] Add DQ subvector inserts

In AVX512f we support 64x2 and 32x8 inserts via matching them to 32x4 and 64x4
respectively.  These are matched by "Alt" Pat<>'s (Alt stands for alternative
VTs).

Since DQ has native support for these intructions, I peeled off the non-"Alt"
part of the baseclass into vinsert_for_size_no_alt. The DQ instructions are
derived from this multiclass.  The "Alt" Pat<>'s are disabled with DQ.

Fixes <rdar://problem/18426089>

llvm-svn: 219874