[X86][AVX] Better support for the variable mask form of VPERMILPD/VPERMILPS

The variable mask form of VPERMILPD/VPERMILPS were only partially implemented, with much of it still performed as an intrinsic.

This patch properly defines the instructions in terms of X86ISD::VPERMILPV, permitting the opcode to be easily combined as a target shuffle.

Differential Revision: http://reviews.llvm.org/D17681

llvm-svn: 262635