[AVX512] Fix copy-and-paste bugs in vpermil

1) i512mem -> f512mem (this is the packed FP input being permuted)
2) element size is 64 bits in EVEX_CD8 for PD.

(A good illustration why X86VectorVTInfo is useful)
[AVX512] Derive vpermil* from avx512_perm_imm

This used to derive from avx512_pshuf_imm which is confusing.

NFC.  Compared X86.td.expanded.
[AVX512] Clean up avx512_perm_imm to use X86VectorVTInfo

No functionality change.  No change in X86.td.expanded except that we only set
the CD8 attributes for the memory variants.  (This shouldn't be used unless we
have a memory operand.)
[AVX512] Add vpermil variable version

This is implemented via a multiclass that derives from the vperm imm
multiclass.

Fixes <rdar://problem/18426089>