Fix mips64 instruction UNPREDICTABLE bug

Replace `addu` with a `sll` instruction with a definite behavior (`sll` will discard the upper 32 bits of the 64 bits, then do sign extensions, with certain behavior). It won't have any UNPREDICTABLE expectation.

Author: hengwu0

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

@@ -1120,9 +1120,11 @@ func (c *ctxt0) asmout(p *obj.Prog, o *Optab, out []uint32) {
 	case 1: /* mov r1,r2 ==> OR r1,r0,r2 */
 		a := AOR
 		if p.As == AMOVW && c.ctxt.Arch.Family == sys.MIPS64 {
-			a = AADDU // sign-extended to high 32 bits
-		}
-		o1 = OP_RRR(c.oprrr(a), uint32(REGZERO), uint32(p.From.Reg), uint32(p.To.Reg))
+			a = ASLL // sign-extended to high 32 bits
+			o1 = OP_RRR(c.oprrr(a), uint32(p.From.Reg), uint32(REGZERO), uint32(p.To.Reg))
+		} else {
+			o1 = OP_RRR(c.oprrr(a), uint32(REGZERO), uint32(p.From.Reg), uint32(p.To.Reg))
+ 		}
 
 	case 2: /* add/sub r1,[r2],r3 */
 		r := int(p.Reg)