glsl: Make mode_string function globally available

glsl: Make mode_string function globally available

I made this a function (instead of a method of ir_variable) because it
made the change set smaller, and I expect that there will be an overload
that takes an ir_var_mode enum.  Having both functions used the same way
seemed better.

v2: Add missing case for ir_var_system_value.

v3: Change the ir_var_mode_count case to just break.  Move the assertion
and the return outside the switch-statment.  In the unlikely event that
var->mode is an invalid value other than ir_var_mode_count, the
assertion will still fire, and in release builds we won't wind up
returning a garbage pointer.  Suggested by Paul.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>