cmd/compile: repair copy-pasted method comment

Inadvertent copy-pasta in CL
https://go-review.googlesource.com/c/36207/

Is this allowed for 1.9 post-freeze?

Change-Id: I00dd27f9f53babeded3695346986a7a491f8a771
Reviewed-on: https://go-review.googlesource.com/43092
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -224,7 +224,9 @@ func (v *Value) copyInto(b *Block) *Value {
 	return c
 }
 
-// copyInto makes a new value identical to v and adds it to the end of b.
+// copyIntoNoXPos makes a new value identical to v and adds it to the end of b.
+// The copied value receives no source code position to avoid confusing changes
+// in debugger information (the intended user is the register allocator).
 func (v *Value) copyIntoNoXPos(b *Block) *Value {
 	c := b.NewValue0(src.NoXPos, v.Op, v.Type) // Lose the position, this causes line number churn otherwise.
 	c.Aux = v.Aux