XINST_CREATE_sub fails to take sp as first source on A64

On A64 you can encode sub with sp as the first source, yet XINST_CREATE_sub fails to encode:

XNST_CREATE_sub(dcontext, opnd_create_reg(DR_REG_X1), opnd_create_reg(DR_REG_SP),
                             opnd_create_reg(DR_REG_X2));
=>
<Internal Error: Failed to encode instruction: 'sub    %sp %x2 lsl $0x0000000000000000 -> %x1'

It's because only the extended-register form takes sp, while the macro hardcodes the shifted-register form. We should have it pick the extended.