glsl: Make _mesa_print_ir() available from anything including ir.h.

glsl: Make _mesa_print_ir() available from anything including ir.h.

No more forgetting to #include "ir_print_visitor.h" when doing temporary
debug code, or forgetting and leaving it in after removing your temporary
debug code.  Also, available from C code so you don't need to move the
caller to C++ just to call it (see also: ir_to_mesa.cpp).

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>