cmd: replace x[i:][0] expressions with x[i]

Passes toolstash -cmp.

Change-Id: Id504e71ed1f23900e24a9aed25143c94f4d7d50c
Reviewed-on: https://go-review.googlesource.com/21899
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/internal/obj/arm64/asm7.go

@@ -1087,7 +1087,7 @@ func aclass(ctxt *obj.Link, a *obj.Addr) int {
 func oplook(ctxt *obj.Link, p *obj.Prog) *Optab {
 	a1 := int(p.Optab)
 	if a1 != 0 {
-		return &optab[a1-1:][0]
+		return &optab[a1-1]
 	}
 	a1 = int(p.From.Class)
 	if a1 == 0 {

src/cmd/internal/obj/mips/asm0.go

@@ -611,7 +611,7 @@ func oplook(ctxt *obj.Link, p *obj.Prog) *Optab {
 
 	a1 := int(p.Optab)
 	if a1 != 0 {
-		return &optab[a1-1:][0]
+		return &optab[a1-1]
 	}
 	a1 = int(p.From.Class)
 	if a1 == 0 {

src/cmd/internal/obj/s390x/asmz.go

@@ -606,7 +606,7 @@ func aclass(ctxt *obj.Link, a *obj.Addr) int {
 func oplook(ctxt *obj.Link, p *obj.Prog) *Optab {
 	a1 := int(p.Optab)
 	if a1 != 0 {
-		return &optab[a1-1:][0]
+		return &optab[a1-1]
 	}
 	a1 = int(p.From.Class)
 	if a1 == 0 {

src/cmd/internal/obj/x86/asm6.go

@@ -4452,9 +4452,8 @@ func asmins(ctxt *obj.Link, p *obj.Prog) {
 	}
 
 	n := ctxt.AsmBuf.Len()
-	var r *obj.Reloc
 	for i := len(ctxt.Cursym.R) - 1; i >= 0; i-- {
-		r = &ctxt.Cursym.R[i:][0]
+		r := &ctxt.Cursym.R[i]
 		if int64(r.Off) < p.Pc {
 			break
 		}

src/cmd/link/internal/ld/decodesym.go

@@ -17,9 +17,9 @@ import (
 // ../gc/reflect.c stuffs in these.
 
 func decode_reloc(s *LSym, off int32) *Reloc {
-	for i := 0; i < len(s.R); i++ {
+	for i := range s.R {
 		if s.R[i].Off == off {
-			return &s.R[i:][0]
+			return &s.R[i]
 		}
 	}
 	return nil

src/cmd/link/internal/ld/ldelf.go

@@ -774,7 +774,7 @@ func ldelf(f *bio.Reader, pkg string, length int64, pn string) {
 		if sym.sym == nil {
 			continue
 		}
-		sect = &elfobj.sect[sym.shndx:][0]
+		sect = &elfobj.sect[sym.shndx]
 		if sect.sym == nil {
 			if strings.HasPrefix(sym.name, ".Linfo_string") { // clang does this
 				continue