[Codegen] SelectOp doesn't generate unsigned types for two branches

[chhzh123]

As the title mentioned, the SelectOp generates the following HLS code which has ap_uint in the true branch and ap_int in the false branch. It cannot pass the synthesis check.

ap_uint<16> v136 = v133 ? (ap_uint<16>)v134 : (ap_int<16>)v135;