i965/fs: Emit a single ADD instruction for SET_SAMPLE_ID on Gen8+.
i965/fs: Emit a single ADD instruction for SET_SAMPLE_ID on Gen8+.

Gen8+ lifted the register region restriction that an instruction whose
destination spans two registers must have sources that also span two
registers.

Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>
Reviewed-by: Anuj Phogat <e2026475996c48f0b97fd37e84129b3912980428@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler