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.

cranelift/src/cat.rs

@@ -1,6 +1,6 @@
 //! The `cat` sub-command.
 //!
-//! Read a sequence of Cretonne IL files and print them again to stdout. This has the effect of
+//! Read a sequence of Cretonne IR files and print them again to stdout. This has the effect of
 //! normalizing formatting and removing comments.
 
 use cton_reader::parse_functions;