nir/spirv: Fix constant num_elements and allocation
nir/spirv: Fix constant num_elements and allocation

Thanks to the addition of nir_clone, we now have a num_elements field in
nir_constant which we weren't setting.  Also, constants have to be parented
to the variable they initialize, so we have to make a copy.

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler