finish up constants

finish up constants

For whatever reason, if I swap the condition in the if_then_else tests
from < to >= and swap the then/else values, I can use constants in
hsl_to_rgb.  Still don't understand why, but I'll take it.  I suspect it
has something to do with SSE, IEEE, and NaN, but I don't care enough to
speculate any more concretely.

This does that, removes C() and _f, updates some comments, and adds a
guard in build_stages.py to yell if it sees trouble like LCPI40_4...

This reminds me to try -ffast-math soon.  I think that was mostly held
back by constants.

Change-Id: I3f8a37a4d4642f77422ce3261b750061e9e604a3
Reviewed-on: https://skia-review.googlesource.com/14942
Reviewed-by: Herb Derby <herb@google.com>