comp_avg_pred: alignment
comp_avg_pred: alignment

x86 requires 16 byte alignment for some vector loads/stores.

arm does not have the same requirement.

The asserts are still in avg_pred_sse2.c. This just removes them from
the common code.

Change-Id: Ic5175c607a94d2abf0b80d431c4e30c8a6f731b6

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