cmd/compile: fix LocResults formatting

When a LocResults is an empty list, it currently prints as ">".
Make it print "<>".

Change-Id: I0f596791b471d74cd4bbc0059e269708c80592dd
Reviewed-on: https://go-review.googlesource.com/c/go/+/350144
Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -91,8 +91,8 @@ func (t LocPair) String() string {
 type LocResults []Location
 
 func (t LocResults) String() string {
-	s := "<"
-	a := ""
+	s := ""
+	a := "<"
 	for _, r := range t {
 		a += s
 		s = ","