Don't renumber entities in the parser.

This makes it easier to debug testcases:
 - the entity numbers in a .cton file match the entity numbers used
   within Cretonne.
 - serializing and deserializing doesn't cause indices to change.

One disadvantage is that if a .cton file uses sparse entity numbers,
deserializing to the in-memory form doesn't compact it. However, the
text format is not intended to be performance-critical, so this isn't
expected to be a big burden.

cranelift/src/filetest/domtree.rs

@@ -61,7 +61,7 @@ impl SubTest for TestDomtree {
                 for src_ebb in tail.split_whitespace() {
                     let ebb = match context.details.map.lookup_str(src_ebb) {
                         Some(AnyEntity::Ebb(ebb)) => ebb,
-                        _ => return Err(format!("expected EBB: {}", src_ebb)),
+                        _ => return Err(format!("expected defined EBB, got {}", src_ebb)),
                     };
 
                     // Annotations say that `inst` is the idom of `ebb`.