[X86] AVX512: Generate Pat<>'s for the vpermt2* intrinsics via multiclass
[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.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@212222 91177308-0d34-0410-b5e6-96231b3b80d8

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