cmd/internal/obj/mips: use r instead of p.Reg in call to OP_IRR

Change-Id: Id77764ed2d693e632e2a7b4e4638c17e0caf2276
GitHub-Last-Rev: 9ebe28252086ddcd530905eb9cf50b4a66413291
GitHub-Pull-Request: golang/go#30003
Reviewed-on: https://go-review.googlesource.com/c/160427Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -1275,7 +1275,7 @@ func (c *ctxt0) asmout(p *obj.Prog, o *Optab, out []uint32) {
 			r = REGZERO
 		}
 		/* only use 10 bits of trap code */
-		o1 = OP_IRR(c.opirr(p.As), (uint32(v)&0x3FF)<<6, uint32(p.Reg), uint32(p.To.Reg))
+		o1 = OP_IRR(c.opirr(p.As), (uint32(v)&0x3FF)<<6, uint32(r), uint32(p.To.Reg))
 
 	case 16: /* sll $c,[r1],r2 */
 		v := c.regoff(&p.From)