cmd/gc: move flow graph into portable opt

Now there's only one copy of the flow graph construction
and dominator computation, and different optimizations
can attach different annotations to the instructions.

R=ken2
CC=golang-dev
https://codereview.appspot.com/12797045
cmd/6l: fix printing of frame size in TEXT instruction

R=ken2
CC=golang-dev
https://codereview.appspot.com/12827043