Say "IR" instead of "IL".

While the specifics of these terms are debatable, "IR" generally
isn't incorrect in this context, and is the more widely recognized
term at this time.

See also the discussion in #267.

Fixes #267.

lib/filetests/src/test_print_cfg.rs

@@ -1,6 +1,6 @@
 //! The `print-cfg` sub-command.
 //!
-//! Read a series of Cretonne IL files and print their control flow graphs
+//! Read a series of Cretonne IR files and print their control flow graphs
 //! in graphviz format.
 
 use std::borrow::Cow;