before

func addr(s[]int) *int {
        return &s[2]
   10c1c:       e28d0008        add     r0, sp, #8
   10c20:       e5901004        ldr     r1, [r0, #4]
   10c24:       e3a02002        mov     r2, #2
   10c28:       e1510002        cmp     r1, r2
   10c2c:       8a000000        bhi     10c34 <main.addr+0x34>
   10c30:       eb0035e6        bl      1e3d0 <runtime.panicindex>
   10c34:       e5900000        ldr     r0, [r0]
   10c38:       e2800008        add     r0, r0, #8
   10c3c:       e58d0014        str     r0, [sp, #20]
   10c40:       e49df004        pop     {pc}            ; (ldr pc, [sp], #4)

after

func addr(s[]int) *int {
	return &s[2]
   10c1c:       e28d0008        add     r0, sp, #8
   10c20:       e5901004        ldr     r1, [r0, #4]
   10c24:       e3510002        cmp     r1, #2
   10c28:       8a000000        bhi     10c30 <main.addr+0x30>
   10c2c:       eb0035e6        bl      1e3cc <runtime.panicindex>
   10c30:       e5900000        ldr     r0, [r0]
   10c34:       e2800008        add     r0, r0, #8
   10c38:       e58d0014        str     r0, [sp, #20]
   10c3c:       e49df004        pop     {pc}            ; (ldr pc, [sp], #4)

Also, relax gcmp restriction that 2nd operand must be a register. A followup
CL will address the remaining TODO items.

R=rsc, remyoudompheng, minux.ma
CC=golang-dev
https://golang.org/cl/6620064