glsl: Don't copy propagate or tree graft precise values.

This is kind of a hack.  We currently track precise requirements
by decorating ir_variables.  Propagating or grafting the RHS of an
assignment to a precise value into some other expression tree can
lose those decorations.

In the long run, it might be better to replace these ir_variable
decorations with an "exact" decoration on ir_expression nodes,
similar to what NIR does.

In the short run, this is probably good enough.  It preserves
enough information for glsl_to_nir to generate "exact" decorations,
and NIR will then handle optimizing these expressions reasonably.

Fixes ES31-CTS.gpu_shader5.precise_qualifier.

v2: Drop invariant handling, as it shouldn't be necessary (caught
    by Jason Ekstrand).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/compiler/glsl/opt_copy_propagation.cpp

@@ -331,7 +331,8 @@ ir_copy_propagation_visitor::add_copy(ir_assignment *ir)
 	 ir->condition = new(ralloc_parent(ir)) ir_constant(false);
 	 this->progress = true;
       } else if (lhs_var->data.mode != ir_var_shader_storage &&
-                 lhs_var->data.mode != ir_var_shader_shared) {
+                 lhs_var->data.mode != ir_var_shader_shared &&
+                 lhs_var->data.precise == rhs_var->data.precise) {
 	 entry = new(this->acp) acp_entry(lhs_var, rhs_var);
 	 this->acp->push_tail(entry);
       }

src/compiler/glsl/opt_copy_propagation_elements.cpp

@@ -493,6 +493,9 @@ ir_copy_propagation_elements_visitor::add_copy(ir_assignment *ir)
       }
    }
 
+   if (lhs->var->data.precise != rhs->var->data.precise)
+      return;
+
    entry = new(this->mem_ctx) acp_entry(lhs->var, rhs->var, write_mask,
 					swizzle);
    this->acp->push_tail(entry);

src/compiler/glsl/opt_tree_grafting.cpp

@@ -368,6 +368,9 @@ tree_grafting_basic_block(ir_instruction *bb_first,
           lhs_var->data.mode == ir_var_shader_shared)
          continue;
 
+      if (lhs_var->data.precise)
+         continue;
+
       ir_variable_refcount_entry *entry = info->refs->get_variable_entry(lhs_var);
 
       if (!entry->declaration ||