Use correct size load in vpx_avg_4x4_sse2.

The old version used 64 bit loads, and then ignored the top half
of the result. This can cause asan failures if we read past the end
of a buffer. Switched to using 32 bit loads instead.

Change-Id: I57da127a26f869fb4b4f700b55408f6dc2fbbc1a
2 files changed