Use sk_srgb_to_linear_trunc in SkColorXform_opts

Use sk_srgb_to_linear_trunc in SkColorXform_opts

This gives us a little more control over instruction order, allowing
us to pipeline the muls and get better performance.  Technically,
clang should be able to do this for us anyway...

Performance on HP z620 (201295.jpg):
toSRGB: 371us -> 356us

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2175413002
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/2175413002