radeonsi: use 32_AR for alpha-to-coverage without a color buffer
radeonsi: use 32_AR for alpha-to-coverage without a color buffer

This avoids the fp16 packing instructions.

Reviewed-by: Nicolai Hähnle <0d0be6cf5ad662651be2dc5dfd44aa2b09d4c1cc@amd.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler