runtime: fix instrumentation of slice append for race detection
runtime: fix instrumentation of slice append for race detection

R=golang-dev, iant
CC=golang-dev
http://codereview.appspot.com/6819107

apparentlymart/golang,dsriram/golang,tv42/old-go,tv42/old-go,tv42/old-go,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,gnanderson/go,dsriram/golang,dsriram/golang,gnanderson/go,dsriram/golang,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,gnanderson/go,dsriram/golang,gnanderson/go