cmd/compile: update stale comment in prove

This is a follow-up CL to https://golang.org/cl/170118, updating a comment made
incorrect by that CL.

Change-Id: I5a29cfae331fbbbb36c96d96f9e4949393a5942d
Reviewed-on: https://go-review.googlesource.com/c/go/+/170123Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/ssa/prove.go

@@ -175,8 +175,7 @@ type factsTable struct {
 	lens map[ID]*Value
 	caps map[ID]*Value
 
-	// zero is a reference to the zero-valued constant assigned or created
-	// during the len/cap sweep that begins prove.
+	// zero is a zero-valued constant
 	zero *Value
 }