Instead of always leaving the work to the generic legalizer when

there is no support for native 256-bit shuffles, be more smart in some
cases, for example, when you can extract specific 128-bit parts and use
regular 128-bit shuffles for them. Example:

For this shuffle:
  shufflevector <4 x i64> %a, <4 x i64> %b, <4 x i32>
                <i32 1, i32 0, i32 7, i32 6>

This was expanded to:
  vextractf128  $1, %ymm1, %xmm2
  vpextrq $0, %xmm2, %rax
  vmovd %rax, %xmm1
  vpextrq $1, %xmm2, %rax
  vmovd %rax, %xmm2
  vpunpcklqdq %xmm1, %xmm2, %xmm1
  vpextrq $0, %xmm0, %rax
  vmovd %rax, %xmm2
  vpextrq $1, %xmm0, %rax
  vmovd %rax, %xmm0
  vpunpcklqdq %xmm2, %xmm0, %xmm0
  vinsertf128 $1, %xmm1, %ymm0, %ymm0
  ret

Now we get:
  vshufpd $1, %xmm0, %xmm0, %xmm0
  vextractf128  $1, %ymm1, %xmm1
  vshufpd $1, %xmm1, %xmm1, %xmm1
  vinsertf128 $1, %xmm1, %ymm0, %ymm0

llvm-svn: 137733