[AVX512] Fix typo

F->I in VEXTRACTF32x4rr.
[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>
[AVX512] Make vextract*x4/vinsert*x4 tests check for the index as well

Extend test so that it provides coverage for the next commit.
[AVX512] Pull pattern for subvector extract into the instruction definition

No functional change.

I initially thought that pulling the Pat<> into the instruction pattern was
not possible because it was doing a transform on the index in order to convert
it from a per-element (extract_subvector) index into a per-chunk (vextract*x4)
index.

Turns out this also works inside the pattern because the vextract_extract
PatFrag has an OperandTransform EXTRACT_get_vextract{128,256}_imm, so the
index in $idx goes through the same conversion.

The existing test CodeGen/X86/avx512-insert-extract.ll extended in the
previous commit provides coverage for this change.