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/cretonne/meta/base/formats.py

@@ -2,7 +2,7 @@
 The cretonne.formats defines all instruction formats.
 
 Every instruction format has a corresponding `InstructionData` variant in the
-Rust representation of cretonne IL, so all instruction formats must be defined
+Rust representation of Cretonne IR, so all instruction formats must be defined
 in this module.
 """
 from __future__ import absolute_import