variance_neon: sync variance*() w/c,sse2
variance_neon: sync variance*() w/c,sse2

removes some unnecessary casts and adds a few explicit uint32 ones for
larger sizes to quiet -Wshorten-64-to-32 warnings

ported from libvpx:
e372bfd5a variance_neon: sync variance*() w/c,sse2

Change-Id: I63c5fce8e62c426d5cf5c10a66a113c119a43518

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,smarter/aom,smarter/aom,GrokImageCompression/aom,smarter/aom,smarter/aom,smarter/aom,smarter/aom,GrokImageCompression/aom,GrokImageCompression/aom