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/reader/src/parser.rs

@@ -1782,7 +1782,7 @@ impl<'a> Parser<'a> {
                     // explicit type specified. Look up `ctrl_value` to see if it was defined
                     // already.
                     // TBD: If it is defined in another block, the type should have been
-                    // specified explicitly. It is unfortunate that the correctness of IL
+                    // specified explicitly. It is unfortunate that the correctness of IR
                     // depends on the layout of the blocks.
                     let ctrl_src_value = inst_data
                         .typevar_operand(&ctx.function.dfg.value_lists)