AR30 is optimized with 3 techniques
1. vpmulhuw is used to replicate 8 bits to 10 bits.
2. Two channels are processed at a time.  R and B, and A and G.
3. vpshufb is used to shift and mask 2 channels of R and B

Red Blue
With the 8 bit value in the upper bits, vpmulhuw by (1024+4) will produce a 10
bit value in the low 10 bits of each 16 bit value. This is whats wanted for the
blue channel. The red needs to be shifted 4 left, so multiply by (1024+4)*16 for
red.

Alpha Green
Alpha and Green are already in the high bits so vpand can zero out the other
bits, keeping just 2 upper bits of alpha and 8 bit green. The same multiplier
could be used for Green - (1024+4) putting the 10 bit green in the lsb.  Alpha
would be a simple multiplier to shift it into position.  It wants a gap of 10
above the green.  Green is 10 bits, so there are 6 bits in the low short.  4
more are needed, so a multiplier of 4 gets the 2 bits into the upper 16 bits,
and then a shift of 4 is a multiply of 16, so (4*16) = 64.  Then shift the
result left 10 to position the A and G channels.

Bug: libyuv:751
Test: ARGBToAR30_Opt
Change-Id: Ie4f20dce18203bae7b75acb1fd5232db8a8a4f11
Reviewed-on: https://chromium-review.googlesource.com/820046
Commit-Queue: Frank Barchard <fbarchard@chromium.org>
Reviewed-by: Cheng Wang <wangcheng@google.com>