Fix typos of "variable" as "varaible"

One of these was just in a comment. But ther other was in an enum
tag, (which is apparently not being used anywhere yet).

ir.h

@@ -134,7 +134,7 @@ enum ir_variable_mode {
    ir_var_inout
 };
 
-enum ir_varaible_interpolation {
+enum ir_variable_interpolation {
    ir_var_smooth = 0,
    ir_var_flat,
    ir_var_noperspective

ir_hierarchical_visitor.h

@@ -93,7 +93,7 @@ public:
     * nodes can always be handled as variable declarations.  Code that used
     * non-hierarchical visitors had to set an "in a dereference" flag to
     * determine how to handle an ir_variable.  By forcing the visitor to
-    * handle the ir_variable within the ir_dereference_varaible visitor, this
+    * handle the ir_variable within the ir_dereference_variable visitor, this
     * kludge can be avoided.
     *
     * In addition, I can envision no use for having separate enter and leave