Use Sk4x4f in srcover_srgb_dst_1.

I've also pulled out the common parts shared with sRGB srcover_n,
and rearranged to make the similarities a bit more clear.

This speeds up about 25%.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1829513002

Review URL: https://codereview.chromium.org/1829513002
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