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/frontend/src/ssa.rs

@@ -231,7 +231,7 @@ fn emit_zero(ty: Type, mut cur: FuncCursor) -> Value {
     }
 }
 /// The following methods are the API of the SSA builder. Here is how it should be used when
-/// translating to Cretonne IL:
+/// translating to Cretonne IR:
 ///
 /// - for each sequence of contiguous instructions (with no branches), create a corresponding
 ///   basic block with `declare_ebb_body_block` or `declare_ebb_header_block` depending on the