if !isAmount {
			return errors.New("invalid register extension")
		}
		switch ext {
		case "UXTB":
			if a.Type == obj.TYPE_MEM {
				return errors.New("invalid shift for the register offset addressing mode")
			}
			a.Reg = arm64.REG_UXTB + Rnum
		case "UXTH":
			if a.Type == obj.TYPE_MEM {
				return errors.New("invalid shift for the register offset addressing mode")
			}

<code>(R2)(R0)</code>:
The location at <code>R0</code> plus <code>R2</code>.
</li>

<li>
<code>R0.UXTB</code>
<br>
<code>R0.UXTB&lt;&lt;imm</code>:
<code>UXTB</code>: extract an 8-bit value from the low-order bits of <code>R0</code> and zero-extend it to the size of <code>R0</code>.
<code>R0.UXTB&lt;&lt;imm</code>: left shift the result of <code>R0.UXTB</code> by <code>imm</code> bits.
The <code>imm</code> value can be 0, 1, 2, 3, or 4.

	CLS R15, ZR                                // ff15c0da
	CLZW R1, R14                               // 2e10c05a
	CLZ R21, R9                                // a912c0da
	CMNW R21.UXTB<<4, R15                      // ff11352b
	CMN R0.UXTW<<4, R16                        // 1f5220ab
	CMNW R13>>8, R9                            // 3f214d2b
	CMN R6->17, R3                             // 7f4486ab

	ANDS	$1, R0, RSP                                      // ERROR "illegal combination"
	ADDSW	R7->32, R14, R13                                 // ERROR "shift amount out of range 0 to 31"
	ADD	R1.UXTB<<5, R2, R3                               // ERROR "shift amount out of range 0 to 4"
	ADDS	R1.UXTX<<7, R2, R3                               // ERROR "shift amount out of range 0 to 4"

	ADD	$0x3fffffffc000, R5             // ADD	$70368744161280, R5               // fb7f72b2a5001b8b
	ADD	R1>>11, R2, R3
	ADD	R1<<22, R2, R3
	ADD	R1->33, R2, R3
	AND	R1@>33, R2, R3
	ADD	R1.UXTB, R2, R3                 // 4300218b
	ADD	R1.UXTB<<4, R2, R3              // 4310218b
	ADD	R2, RSP, RSP                    // ff63228b
	ADD	R2.SXTX<<1, RSP, RSP            // ffe7228b
	ADD	ZR.SXTX<<1, R2, R3              // 43e43f8b