Extend reg_resize_to_opsz to support SIMD registers

Feature request to extend reg_resize_to_opsz and support SIMD registers.

We need to easily switch between different sizes of a SIMD register, e.g., from DR_REG_XMM0 to DR_REG_ZMM0 or to DR_REG_YMM0 back to DR_REG_XMM0.