glsl: Check the size of ir_print_visitor's mode[] array with STATIC_ASSERT.

ir_print_visitor::visit(ir_variable *)'s mode[] array needs to match
the declaration of the enum ir_variable_mode.  It's hard to verify
that at compile time, but at least we can use a STATIC_ASSERT to make
sure it's the right size.

This required adding ir_var_mode_count to the enum.