cmd/internal/obj/mips: use LR as temporary register in RET

On mips/mips64, for non-leaf function, RET is assembled as
	MOV (SP), R4       // load saved LR
	ADD $framesize, SP
	JMP (R4)

This clobbers R4 unnecessarily. Use the link register as
temporary instead.

Probably for Go 1.11.

Change-Id: I2209db7be11074ed2e0e0829cace95ebfb709e9f
Reviewed-on: https://go-review.googlesource.com/79016
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Austin Clements <austin@google.com>

src/cmd/internal/obj/mips/obj0.go

@@ -495,10 +495,7 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 			p.From.Offset = 0
 			p.From.Reg = REGSP
 			p.To.Type = obj.TYPE_REG
-			p.To.Reg = REG_R4
-			if retSym != nil { // retjmp from non-leaf, need to restore LINK register
-				p.To.Reg = REGLINK
-			}
+			p.To.Reg = REGLINK
 
 			if autosize != 0 {
 				q = c.newprog()
@@ -524,7 +521,7 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 			} else {
 				q1.To.Type = obj.TYPE_MEM
 				q1.To.Offset = 0
-				q1.To.Reg = REG_R4
+				q1.To.Reg = REGLINK
 			}
 			q1.Mark |= BRANCH
 			q1.Spadj = +autosize