[AVX512] Refactor subvector extracts

[AVX512] Refactor subvector extracts

No functional change.

These are now implemented as two levels of multiclasses heavily relying on the
new X86VectorVTInfo class.  The multiclass at the first level that is called
with float or int provides the 128 or 256 bit subvector extracts.  The second
level provides the register and memory variants and some more Pat<>s.

I've compared the td.expanded files before and after.  One change is that
ExeDomain for 64x4 is SSEPackedDouble now.  I think this is correct, i.e. a
bugfix.

(BTW, this is the change that was blocked on the recent tablegen fix.  The
class-instance values X86VectorVTInfo inside vextract_for_type weren't
properly evaluated.)

Part of <rdar://problem/17688758>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218478 91177308-0d34-0410-b5e6-96231b3b80d8