optimize src mode (opaque src in srcover) singleton with aa (e.g. a8 text mask)

optimize src mode (opaque src in srcover) singleton with aa (e.g. a8 text mask)

Before:
   8/8   MB	1	9.09ms	10.8ms	10.3ms	11.5ms	9%	█▆▆▁▁▁▂▅█▆	nonrendering	xfer4f_srcover_aa_1_opaque_linear
   8/8   MB	1	10.2ms	12.1ms	11.7ms	13.2ms	9%	▅▇▁▂▁▄█▆▅▆	nonrendering	xfer4f_srcover_aa_1_opaque_srgb

After:
   8/8   MB	1	1.6ms	1.68ms	1.73ms	2.17ms	10%	▄▄█▁▃▁▁▂▁▁	nonrendering	xfer4f_srcover_aa_1_opaque_linear
   8/8   MB	1	3.13ms	3.62ms	3.97ms	5.81ms	21%	█▃▁▂▆▂▂▂▃▂	nonrendering	xfer4f_srcover_aa_1_opaque_srgb

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1664713003

Review URL: https://codereview.chromium.org/1664713003