[AVX-512] Fix DecodeVPERMV3Mask to handle cases where the constant pool entry...

[AVX-512] Fix DecodeVPERMV3Mask to handle cases where the constant pool entry has a different type than the shuffle itself.

Summary: This is especially important for 32-bit targets with 64-bit shuffle elements.This is similar to how PSHUFB and VPERMIL handle the same problem.

Reviewers: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D25666

llvm-svn: 284451