[LCG] Cleanup various places where comments said `SCC` but meant

`RefSCC`.

Also improve the comments surrounding the lazy post-order iterator as
they had grown stale since the RefSCC/SCC split.

I'm sure there are more comments that need updating here, but I saw and
fixed these and didn't want to lose them. I've not gotten to doing
a really complete audit of every comment yet.

llvm-svn: 283987