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[X86, AVX] improve insertion into zero element of 256-bit vector

Description

[X86, AVX] improve insertion into zero element of 256-bit vector

This patch allows AVX blend instructions to handle insertion into the low
element of a 256-bit vector for the appropriate data types.

For f32, instead of:

vblendps	$1, %xmm1, %xmm0, %xmm1 ## xmm1 = xmm1[0],xmm0[1,2,3]
vblendps	$15, %ymm1, %ymm0, %ymm0 ## ymm0 = ymm1[0,1,2,3],ymm0[4,5,6,7]

we get:

vblendps	$1, %ymm1, %ymm0, %ymm0 ## ymm0 = ymm1[0],ymm0[1,2,3,4,5,6,7]

For f64, instead of:

vmovsd	%xmm1, %xmm0, %xmm1     ## xmm1 = xmm1[0],xmm0[1]
vblendpd	$3, %ymm1, %ymm0, %ymm0 ## ymm0 = ymm1[0,1],ymm0[2,3]

we get:

vblendpd	$1, %ymm1, %ymm0, %ymm0 ## ymm0 = ymm1[0],ymm0[1,2,3]

For the hardware-neglected integer data types, I left a TODO comment in the
code and added regression tests for a follow-on patch.

Differential Revision: http://reviews.llvm.org/D8609

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