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/compile.rs

@@ -1,6 +1,4 @@
-//! CLI tool to compile cretonne IL into native code.
-//!
-//! Reads IR files into Cretonne IL and compiles it.
+//! CLI tool to read Cretonne IR files and compile them into native code.
 
 use cton_reader::parse_test;
 use std::path::PathBuf;