New APIs:
(*BasicBlock).{Idom,Dominees,Dominates}
(*Function).DomPreorder
Messy but systematic refactoring of domNode:
- renamed "domInfo".
- embedded directly in BasicBlock, not as pointer. Block field removed.
- Level field removed; was unused.
- Working state of LT algorithm now in its own type.
{semi,parent,ancestor} fields moved into it.
- remaining fields made private; accessors added.
- use 32-bit ints for pre/postorder numbers.
- allocate LT working space (5 copies of fn.Blocks) contiguously.
dom.go is simpler but somewhat more verbose.
Also:
- we always build the domtree now---yet memory usage is down 5%.
- number the Recover block too.
- add sanity check for DomPreorder.
R=gri
CC=golang-dev
https://golang.org/cl/37230043
b5016cbbbd