Most significant in mandelbrot, from avoiding MOVSD between registers,
but there are others.
R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6258063
Surprise! The C code is using floating point values for its counters.
Its off the critical path, but the Go code and C code are supposed to
be as similar as possible to make comparisons meaningful.
It doesn't have a significant effect.
R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6260058
Moving panic out of line speeds up fannkuch almost a factor of two.
Changes to bitwhacking code affect mandelbrot badly.
R=golang-dev, bradfitz, rsc, r
CC=golang-dev
https://golang.org/cl/6258056
