This patch moves the C++/SSE2/NEON implementations of convolve functions
into the same place and uses SkOpts framework.
Also some indentation fix.
BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2500113004
CQ_INCLUDE_TRYBOTS=master.client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot
Review-Url: https://codereview.chromium.org/2500113004