nir: Avoid coalescing vars created by lower_io_to_temporaries

Right now nir_copy_prop_vars is effectively undoing
nir_lower_io_to_temporaries for inputs by propagating the original
variable through the copy created in lower_io_to_temporaries. A
theoretical variable coalescing pass would have the same issue with
output variables, although that doesn't exist yet. To fix this, add a
new bit to nir_variable, and disable copy propagation when it's set.

This doesn't seem to affect any drivers now, probably since since no one
uses lower_io_to_temporaries for inputs as well as copy_prop_vars, but
it will fix radv once we flip on lower_io_to_temporaries for fs inputs.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>

src/compiler/nir/nir.h

@@ -227,6 +227,17 @@ typedef struct nir_variable {
       unsigned patch:1;
       unsigned invariant:1;
 
+      /**
+       * Can this variable be coalesced with another?
+       *
+       * This is set by nir_lower_io_to_temporaries to say that any
+       * copies involving this variable should stay put. Propagating it can
+       * duplicate the resulting load/store, which is not wanted, and may
+       * result in a load/store of the variable with an indirect offset which
+       * the backend may not be able to handle.
+       */
+      unsigned cannot_coalesce:1;
+
       /**
        * When separate shader programs are enabled, only input/outputs between
        * the stages of a multi-stage separate program can be safely removed