glsl2: Add and use new variable mode ir_var_temporary

glsl2: Add and use new variable mode ir_var_temporary

This is quite a large patch because breaking it into smaller pieces
would result in the tree being intermitently broken.  The big changes
are:

    * Add the ir_var_temporary variable mode

    * Change the ir_variable constructor to take the mode as a
      parameter and correctly specify the mode for all ir_varables.

    * Change the linker to not cross validate ir_var_temporary
      variables.

    * Change the linker to pull all ir_var_temporary variables from
      global scope into 'main'.