[X86] AVX512: Add writemask variants for vperm*2*

This includes assembler and codegen support (see the new tests in
avx512-encodings.s and avx512-shuffle.ll).

<rdar://problem/17492620>
[X86] AVX512: Generate Pat<>'s for the vpermt2* intrinsics via multiclass

This new multiclass, avx512_perm_table_3src derives from the current one and
provides the Pat<>.  The next patch will add another Pat<> that uses the
writemask.

Note that I dropped the type annotation from the intrinsic call, i.e.: (v16f32
VR512:$src1) -> R512:$src1.  I think that this should be fine (at least many
intrinsic calls don't provide them) and it greatly reduces the number of
template arguments.
[X86] AVX512: Allow writemask argument in vpermt* intrinsics