glsl: Add a method to get precision from a deref instruction

Adds ir_dereference::precision(). For a normal variable dereference,
the precision comes from the variable. For a record member it comes
from the field within the record. For an array it can come from
either, depending on where the underlying array is stored. The method
recursively walks the derefs until it finds one of the first two.

Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3885>

src/compiler/glsl/ir.h

@@ -2047,6 +2047,12 @@ public:
     */
    virtual ir_variable *variable_referenced() const = 0;
 
+   /**
+    * Get the precision. This can either come from the eventual variable that
+    * is dereferenced, or from a record member.
+    */
+   virtual int precision() const = 0;
+
 protected:
    ir_dereference(enum ir_node_type t)
       : ir_rvalue(t)
@@ -2076,6 +2082,11 @@ public:
       return this->var;
    }
 
+   virtual int precision() const
+   {
+      return this->var->data.precision;
+   }
+
    virtual ir_variable *whole_variable_referenced()
    {
       /* ir_dereference_variable objects always dereference the entire
@@ -2124,6 +2135,16 @@ public:
       return this->array->variable_referenced();
    }
 
+   virtual int precision() const
+   {
+      ir_dereference *deref = this->array->as_dereference();
+
+      if (deref == NULL)
+         return GLSL_PRECISION_NONE;
+      else
+         return deref->precision();
+   }
+
    virtual void accept(ir_visitor *v)
    {
       v->visit(this);
@@ -2159,6 +2180,13 @@ public:
       return this->record->variable_referenced();
    }
 
+   virtual int precision() const
+   {
+      glsl_struct_field *field = record->type->fields.structure + field_idx;
+
+      return field->precision;
+   }
+
    virtual void accept(ir_visitor *v)
    {
       v->visit(this);