ac/nir: rewrite ac_nir_lower_ps epilog to fix dual src blending with mono PS

Unigine Heaven with AMD_DEBUG=mono has incorrect rendering on gfx11 because it doesn't set nir_io_semantics::dual_source_blend_index for the second output, resulting in garbage asm.

Instead of trying to find out what's wrong, I decided to rewrite this to make it the same as the LLVM IR path. It simplifies the code and fixes Unigine Heaven with AMD_DEBUG=mono.