Add support for 256-bit versions of VPERMIL instruction. This is a new

instruction introduced in AVX, which can operate on 128 and 256-bit vectors.
It considers a 256-bit vector as two independent 128-bit lanes. It can permute
any 32 or 64 elements inside a lane, and restricts the second lane to
have the same permutation of the first one. With the improved splat support
introduced early today, adding codegen for this instruction enable more
efficient 256-bit code:

Instead of:
  vextractf128  $0, %ymm0, %xmm0
  punpcklbw %xmm0, %xmm0
  punpckhbw %xmm0, %xmm0
  vinsertf128 $0, %xmm0, %ymm0, %ymm1
  vinsertf128 $1, %xmm0, %ymm1, %ymm0
  vextractf128  $1, %ymm0, %xmm1
  shufps  $1, %xmm1, %xmm1
  movss %xmm1, 28(%rsp)
  movss %xmm1, 24(%rsp)
  movss %xmm1, 20(%rsp)
  movss %xmm1, 16(%rsp)
  vextractf128  $0, %ymm0, %xmm0
  shufps  $1, %xmm0, %xmm0
  movss %xmm0, 12(%rsp)
  movss %xmm0, 8(%rsp)
  movss %xmm0, 4(%rsp)
  movss %xmm0, (%rsp)
  vmovaps (%rsp), %ymm0
We get:
  vextractf128  $0, %ymm0, %xmm0
  punpcklbw %xmm0, %xmm0
  punpckhbw %xmm0, %xmm0
  vinsertf128 $0, %xmm0, %ymm0, %ymm1
  vinsertf128 $1, %xmm0, %ymm1, %ymm0
  vpermilps $85, %ymm0, %ymm0

llvm-svn: 135662