AArch64: V28 register mismangled as the stolen X28 register

It seems the `instr_uses_reg(instr, dr_reg_stolen)` is not returning the correct result for instructions that use SIMD registers. Presumably there is a bug in the decoder.

As a result, I've found the unessessary mangles in the DynamoRIO AArch64 7.0.0-RC1 release:

```
fmov d28, d12
```
is manged to
```
str x1, [x28,#8]
mov x1, x28
ldr x28, [x28,#48]
fmov d28, d12
str x28, [x1,#48]
mov x28, x1
ldr x1, [x28,#8]
```

Start with instr_uses_reg(instr, dr_reg_stolen) and the AArch64 decoder, reproducing the reported fmov d28, d12 case from the issue. Trace how SIMD register operands are identified and verify that V28 is not treated as the stolen X28 register; done means the unnecessary mangling is absent and the register-use result is correct.

Written by the indexing model from the issue text.