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
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