variance_avx2: sync variance functions with c-code

add missing int64 -> uint32 cast; quiets -Wshorten-64-to-32 warnings

Change-Id: I4850b36e18dc8b399108342be4bfe0b684aefb78
(cherry picked from commit 6acd061aad8cf62000cc9117390d0c94581a8591)
1 file changed