Remove the concept of non-dense jump tables.

WebAssembly doesn't have non-dense jump tables, and higher-level users
are better served by the facilities in lib/frontend/src/switch.rs for
working with non-dense switches.

This eliminates the concept of "absent" jump table entries, which
were represented as "0" in the text format.

Also, jump table contents are now enclosed in `[` and `]`, so that
we can unambiguously display empty jump tables. Previously, empty jump
tables were displayed as if they had a single absent entry.

lib/codegen/src/dominator_tree.rs

@@ -347,8 +347,8 @@ impl DominatorTree {
             match func.dfg.analyze_branch(inst) {
                 BranchInfo::SingleDest(succ, _) => self.push_if_unseen(succ),
                 BranchInfo::Table(jt, dest) => {
-                    for (_, succ) in func.jump_tables[jt].entries() {
-                        self.push_if_unseen(succ);
+                    for succ in func.jump_tables[jt].iter() {
+                        self.push_if_unseen(*succ);
                     }
                     if let Some(dest) = dest {
                         self.push_if_unseen(dest);