i965/fs: Add builder emit method taking a variable number of source registers.
i965/fs: Add builder emit method taking a variable number of source registers.

And start using it in fs_builder::LOAD_PAYLOAD().  This will be used
to emit logical send message opcodes which have an unusually large
number of arguments.

Reviewed-by: Jason Ekstrand <b186559eaedf698788888c8a2559b4efb4db10d6@intel.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler