image/draw benchmarks show <1% change for the fast paths.
The slow paths got worse by 1-4%, but they're the slow paths.
I don't care so much about them, and presumably compiler improvements
could claw it back.

IIUC 6g's inlining is enabled by default now.

benchmark                        old ns/op    new ns/op    delta
draw.BenchmarkFillOver             2988384      2999624   +0.38%
draw.BenchmarkFillSrc               153141       153262   +0.08%
draw.BenchmarkCopyOver             2155756      2170831   +0.70%
draw.BenchmarkCopySrc                72591        72646   +0.08%
draw.BenchmarkNRGBAOver            2487372      2491576   +0.17%
draw.BenchmarkNRGBASrc             1361306      1409180   +3.52%
draw.BenchmarkYCbCr                2540712      2562359   +0.85%
draw.BenchmarkGlyphOver            1004879      1023308   +1.83%
draw.BenchmarkRGBA                 8746670      8844455   +1.12%
draw.BenchmarkGenericOver         31860960     32512960   +2.05%
draw.BenchmarkGenericMaskOver     16369060     16435720   +0.41%
draw.BenchmarkGenericSrc          13128540     13127810   -0.01%
draw.BenchmarkGenericMaskSrc      30059300     28883210   -3.91%

R=r, gri
CC=golang-dev, rsc
https://golang.org/cl/5536059