cmd/compile/internal/ssa: fix partsByVarOffset.Less method

Fix duplicated index in LHS and RHS of the < operator.

Found using https://go-critic.github.io/overview#dupSubExpr-ref

Change-Id: I9a5a40bbd436b32e8117579a01bc50afe3608c97
Reviewed-on: https://go-review.googlesource.com/122776Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -765,7 +765,7 @@ type partsByVarOffset struct {
 
 func (a partsByVarOffset) Len() int { return len(a.slotIDs) }
 func (a partsByVarOffset) Less(i, j int) bool {
-	return varOffset(a.slots[a.slotIDs[i]]) < varOffset(a.slots[a.slotIDs[i]])
+	return varOffset(a.slots[a.slotIDs[i]]) < varOffset(a.slots[a.slotIDs[j]])
 }
 func (a partsByVarOffset) Swap(i, j int) { a.slotIDs[i], a.slotIDs[j] = a.slotIDs[j], a.slotIDs[i] }