cmd/5g: avoid temporary during constant OINDEX

func addr(s[]int) *int {
	return &s[2]
}

--- prog list "addr" ---
0000 (/home/dfc/src/addr.go:5) TEXT     addr+0(SB),$0-16
0001 (/home/dfc/src/addr.go:6) MOVW     $s+0(FP),R0
0002 (/home/dfc/src/addr.go:6) MOVW     4(R0),R1
0003 (/home/dfc/src/addr.go:6) MOVW     $2,R2
0004 (/home/dfc/src/addr.go:6) CMP      R2,R1,
0005 (/home/dfc/src/addr.go:6) BHI      ,7(APC)
0006 (/home/dfc/src/addr.go:6) BL       ,runtime.panicindex+0(SB)
0007 (/home/dfc/src/addr.go:6) MOVW     0(R0),R0
0008 (/home/dfc/src/addr.go:6) MOVW     $8,R1
0009 (/home/dfc/src/addr.go:6) ADD      R1,R0
0010 (/home/dfc/src/addr.go:6) MOVW     R0,.noname+12(FP)
0011 (/home/dfc/src/addr.go:6) RET      ,

becomes

--- prog list "addr" ---
0000 (/home/dfc/src/addr.go:5) TEXT     addr+0(SB),$0-16
0001 (/home/dfc/src/addr.go:6) MOVW     $s+0(FP),R0
0002 (/home/dfc/src/addr.go:6) MOVW     4(R0),R1
0003 (/home/dfc/src/addr.go:6) MOVW     $2,R2
0004 (/home/dfc/src/addr.go:6) CMP      R2,R1,
0005 (/home/dfc/src/addr.go:6) BHI      ,7(APC)
0006 (/home/dfc/src/addr.go:6) BL       ,runtime.panicindex+0(SB)
0007 (/home/dfc/src/addr.go:6) MOVW     0(R0),R0
0008 (/home/dfc/src/addr.go:6) ADD      $8,R0
0009 (/home/dfc/src/addr.go:6) MOVW     R0,.noname+12(FP)
0010 (/home/dfc/src/addr.go:6) RET      ,

R=rsc, remyoudompheng, minux.ma
CC=golang-dev
https://golang.org/cl/6590056

src/cmd/5g/cgen.c

@@ -733,11 +733,7 @@ agen(Node *n, Node *res)
 			}
 
 			nodconst(&n2, types[tptr], v*w);
-			regalloc(&n4, n2.type, N);
-			gmove(&n2, &n4);
-			gins(optoas(OADD, types[tptr]), &n4, &n3);
-			regfree(&n4);
-
+			gins(optoas(OADD, types[tptr]), &n2, &n3);
 			gmove(&n3, res);
 			regfree(&n3);
 			break;