radeonsi: don't overwrite the scratch offset in shader prologs
radeonsi: don't overwrite the scratch offset in shader prologs

Prologs only look at num_input_sgprs.

Reviewed-by: Bas Nieuwenhuizen <1d31d94f30d40df7951505d1034e1e923d02ec49@basnieuwenhuizen.nl>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler