cmd/compile: tweak ssa.html colors

cmd/compile: tweak ssa.html colors

Make yellow the last highlight color rather than the first.
Yellow is also the color that Chrome uses to highlight
search results, which can be confusing.
Also, when Night Shift is on on macOS,
yellow highlighting is completely invisible.
I suppose should be sleeping instead.

Also, remove a completed TODO.

Change-Id: I0eb4439272fad9ccb5fe8e2cf409fdd5dc15b26e
Reviewed-on: https://go-review.googlesource.com/43463
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>