nir: Mark nir_start_block()/nir_impl_last_block() with returns_nonnull.
nir: Mark nir_start_block()/nir_impl_last_block() with returns_nonnull.

Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler