i965/vec4/nir: Emit single MOV to generate a scalar constant.
i965/vec4/nir: Emit single MOV to generate a scalar constant.

If an immediate is written to multiple channels, we can load it in a
single writemasked MOV.

total instructions in shared programs: 6285144 -> 6261991 (-0.37%)
instructions in affected programs:     718991 -> 695838 (-3.22%)
helped:                                5762

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

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler