[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)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220734 91177308-0d34-0410-b5e6-96231b3b80d8