SunReg should always be AMDGPU::sub0. The 8-bit m0 field for the index is unsigned.
We can guarantee the index non-negative (if the program itself is correct) only when we
start from the very first register in the vector.
The original optimization shifts the base to AMDGPU::sub0 + Offset, which leads to the situation
that the index could be negative to address the registers to the left of the base (Offset). Thus the
optimization is invalid.