[X86] Improved lowering of packed v8i16 vector shifts by non-constant count.

Before this patch, the backend sub-optimally expanded the non-constant shift
count of a v8i16 shift into a sequence of two 'movd' plus 'movzwl'.

With this patch the backend checks if the target features sse4.1. If so, then
it lets the shuffle legalizer deal with the expansion of the shift amount.

Example:
;;
define <8 x i16> @test(<8 x i16> %A, <8 x i16> %B) {
  %shamt = shufflevector <8 x i16> %B, <8 x i16> undef, <8 x i32> zeroinitializer
  %shl = shl <8 x i16> %A, %shamt
  ret <8 x i16> %shl
}
;;

Before (with -mattr=+avx):
  vmovd  %xmm1, %eax
  movzwl  %ax, %eax
  vmovd  %eax, %xmm1
  vpsllw  %xmm1, %xmm0, %xmm0
  retq

Now:
  vpxor  %xmm2, %xmm2, %xmm2
  vpblendw  $1, %xmm1, %xmm2, %xmm1
  vpsllw  %xmm1, %xmm0, %xmm0
  retq