[AVX] Improve insertion of i8 or i16 into low element of 256-bit zero vector

Without this patch, we split the 256-bit vector into halves and produced something like:
	movzwl	(%rdi), %eax
	vmovd	%eax, %xmm0
	vxorps	%xmm1, %xmm1, %xmm1
	vblendps	$15, %ymm0, %ymm1, %ymm0 ## ymm0 = ymm0[0,1,2,3],ymm1[4,5,6,7]

Now, we eliminate the xor and blend because those zeros are free with the vmovd:
        movzwl  (%rdi), %eax
        vmovd   %eax, %xmm0

This should be the final fix needed to resolve PR22685:
https://llvm.org/bugs/show_bug.cgi?id=22685

llvm-svn: 233941