MOVSD only copies the low half of the packed register pair,
while MOVAPD copies both halves.  I assume the internal
register renaming works better with the latter, since it makes
our code run 25% faster.

Before:
mandelbrot 16000
        gcc -O2 mandelbrot.c	28.44u 0.00s 28.45r
        gc mandelbrot	44.12u 0.00s 44.13r
        gc_B mandelbrot	44.17u 0.01s 44.19r

After:
mandelbrot 16000
        gcc -O2 mandelbrot.c	28.22u 0.00s 28.23r
        gc mandelbrot	32.81u 0.00s 32.82r
        gc_B mandelbrot	32.82u 0.00s 32.83r

R=ken2
CC=golang-dev
https://golang.org/cl/6248068