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/src/regalloc/diversion.rs

@@ -15,7 +15,7 @@ use std::vec::Vec;
 
 /// A diversion of a value from its original location to a new register or stack location.
 ///
-/// In IL, a diversion is represented by a `regmove` instruction, possibly a chain of them for the
+/// In IR, a diversion is represented by a `regmove` instruction, possibly a chain of them for the
 /// same value.
 ///
 /// When tracking diversions, the `from` field is the original assigned value location, and `to` is