nir: opt_offsets for load_ubo_vec4 on non-native-integers

@stefandoesinger provided me GLSL for a wine-generated shader for a DX9 game that was failing to compile on the DX9-class r500. It turned out that we were doing addressing math in ALU ops that was expected to be folded into the const-file (uniforms) reference. nir_opt_offsets is a pass to do that, but we needed a base field in load_ubo_vec4 to fold into. With this MR plus !14158 (closed) and !14200 (merged), the shader now compiles.

Fixes: #5781 (closed)