cmd/compile: fix some plive comments

The liveness analysis no longer directly emits PCDATA. Fix stale
comments that say so.

Change-Id: Id26b112ddf4c13a12ebf766f64bf57c68fbfe3ef
Reviewed-on: https://go-review.googlesource.com/67691
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/plive.go

@@ -692,7 +692,7 @@ func (lv *Liveness) epilogue() {
 	for _, b := range lv.f.Blocks {
 		be := lv.blockEffects(b)
 
-		// walk backward, emit pcdata and populate the maps
+		// walk backward, construct maps at each safe point
 		index := int32(be.lastbitmapindex)
 		if index < 0 {
 			// the first block we encounter should have the ATEXT so
@@ -1012,7 +1012,8 @@ Outer:
 	}
 	lv.livevars = lv.livevars[:uniq]
 
-	// Rewrite PCDATA instructions to use new numbering.
+	// Record compacted stack map indexes for each value.
+	// These will later become PCDATA instructions.
 	lv.showlive(nil, lv.livevars[0])
 	pos := 1
 	lv.stackMapIndex = make(map[*ssa.Value]int)