cmd/internal/obj/arm: fix wrong encoding of MUL

The arm assembler incorrectly encodes the following instructions.
"MUL R2, R4" -> 0xe0040492 ("MUL R4, R2, R4")
"MUL R2, R4, R4" -> 0xe0040492 ("MUL R4, R2, R4")

The CL fixes that issue.

fixes #25347

Change-Id: I883716c7bc51c5f64837ae7d81342f94540a58cb
Reviewed-on: https://go-review.googlesource.com/112737Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/asm/internal/asm/testdata/arm.s

@@ -951,13 +951,17 @@ jmp_label_3:
 
 // MUL
 	MUL	R2, R3, R4           // 930204e0
-	MUL	R2, R4               // 920404e0
+	MUL	R2, R4               // 940204e0
+	MUL	R2, R4, R4           // 940204e0
 	MUL.S	R2, R3, R4           // 930214e0
-	MUL.S	R2, R4               // 920414e0
+	MUL.S	R2, R4               // 940214e0
+	MUL.S	R2, R4, R4           // 940214e0
 	MULU	R5, R6, R7           // 960507e0
-	MULU	R5, R7               // 950707e0
+	MULU	R5, R7               // 970507e0
+	MULU	R5, R7, R7           // 970507e0
 	MULU.S	R5, R6, R7           // 960517e0
-	MULU.S	R5, R7               // 950717e0
+	MULU.S	R5, R7               // 970517e0
+	MULU.S	R5, R7, R7           // 970517e0
 	MULLU	R1, R2, (R4, R3)     // 923184e0
 	MULLU.S	R1, R2, (R4, R3)     // 923194e0
 	MULL	R1, R2, (R4, R3)     // 9231c4e0

src/cmd/internal/obj/arm/asm5.go

@@ -2046,16 +2046,6 @@ func (c *ctxt5) asmout(p *obj.Prog, o *Optab, out []uint32) {
 		if r == 0 {
 			r = rt
 		}
-		if rt == r {
-			r = rf
-			rf = rt
-		}
-
-		if false {
-			if rt == r || rf == REGPC&15 || r == REGPC&15 || rt == REGPC&15 {
-				c.ctxt.Diag("%v: bad registers in MUL", p)
-			}
-		}
 
 		o1 |= (uint32(rf)&15)<<8 | (uint32(r)&15)<<0 | (uint32(rt)&15)<<16