cmd/compile/internal/liveness: use slices.Reverse

Now that we're bootstrapping from a toolchain that has the slices
package.

Updates #64751

Change-Id: Id50d76de05e353ef06d64b47ad6400b2b7572205
Reviewed-on: https://go-review.googlesource.com/c/go/+/610775
Auto-Submit: Cuong Manh Le <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

src/cmd/compile/internal/liveness/intervals.go

@@ -49,6 +49,7 @@ package liveness
 import (
 	"fmt"
 	"os"
+	"slices"
 	"strings"
 )
 
@@ -131,11 +132,7 @@ func (c *IntervalsBuilder) setLast(x int) {
 
 func (c *IntervalsBuilder) Finish() (Intervals, error) {
 	// Reverse intervals list and check.
-	// FIXME: replace with slices.Reverse once the
-	// bootstrap version supports it.
-	for i, j := 0, len(c.s)-1; i < j; i, j = i+1, j-1 {
-		c.s[i], c.s[j] = c.s[j], c.s[i]
-	}
+	slices.Reverse(c.s)
 	if err := check(c.s); err != nil {
 		return Intervals{}, err
 	}