[AVX512] Rename arg from Opcode32/64 to Opcode128/256 in vinsert_for_size

It's the W bit that selects between 32 or 64 elt type and not the opcode.  The
opcode selects between the width of the insert (128 or 256).
[AVX512] Two new attributes in X86VectorVTInfo for subvector insert

The new attributes are NumElts and the CD8TupleForm.  This prepares the code
to enable x8 and x2 inserts.

NFC, no change in X86.td.expanded except for the new attributes.
[AVX512] Fix test to produce a defined value

We're inserting into a 8 wide vector, so the index should be < 8.
[AVX512] Add SKX testing to avx512-insert-extract.ll

This is in preparation to adding DQ subvector inserts to this testcase.
[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>