glsl: Move 'foo = foo;' optimization to opt_dead_code_local

glsl: Move 'foo = foo;' optimization to opt_dead_code_local

The optimization as done in opt_copy_propagation would have to be
removed in the next patch.  If we just eliminate that optimization
altogether, shader-db results, even on platforms that use NIR, are hurt
quite substantially.  I have not investigated why NIR isn't picking up
the slack here.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Cc: Jason Ekstrand <jason@jlekstrand.net>