* inform garbage collector about memory with no pointers in it

1.9s	gcc reverse-complement.c

reverse-complement.go
4.5s / 3.5s	original, with/without bounds checks
3.5s / 3.3s	bounds check reduction
3.3s / 2.8s	smarter garbage collector
2.6s / 2.3s		assembler bytes.IndexByte
2.5s / 2.1s	even smarter garbage collector (this CL)

R=r
https://golang.org/cl/165064