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/frontend/src/frontend.rs

@@ -152,12 +152,11 @@ impl<'short, 'long> InstBuilderBase<'short> for FuncInstBuilder<'short, 'long> {
                             .jump_tables
                             .get(table)
                             .expect("you are referencing an undeclared jump table")
-                            .entries()
-                            .map(|(_, ebb)| ebb)
-                            .filter(|dest_ebb| unique.insert(*dest_ebb))
+                            .iter()
+                            .filter(|&dest_ebb| unique.insert(*dest_ebb))
                         {
                             self.builder.func_ctx.ssa.declare_ebb_predecessor(
-                                dest_ebb,
+                                *dest_ebb,
                                 self.builder.position.basic_block.unwrap(),
                                 inst,
                             )
@@ -336,11 +335,6 @@ impl<'a> FunctionBuilder<'a> {
         self.func.create_jump_table(data)
     }
 
-    /// Inserts an entry in a previously declared jump table.
-    pub fn insert_jump_table_entry(&mut self, jt: JumpTable, index: usize, ebb: Ebb) {
-        self.func.insert_jump_table_entry(jt, index, ebb)
-    }
-
     /// Creates a stack slot in the function, to be used by `stack_load`, `stack_store` and
     /// `stack_addr` instructions.
     pub fn create_stack_slot(&mut self, data: StackSlotData) -> StackSlot {

lib/frontend/src/ssa.rs

@@ -672,8 +672,8 @@ impl SSABuilder {
                 }
 
                 for old_dest in func.jump_tables[jt].as_mut_slice() {
-                    if *old_dest == PackedOption::from(dest_ebb) {
-                        *old_dest = PackedOption::from(middle_ebb);
+                    if *old_dest == dest_ebb {
+                        *old_dest = middle_ebb;
                     }
                 }
                 let mut cur = FuncCursor::new(func).at_bottom(middle_ebb);
@@ -1005,7 +1005,7 @@ mod tests {
         // Here is the pseudo-program we want to translate:
         //
         // function %f {
-        // jt = jump_table ebb2, 0, ebb1
+        // jt = jump_table [ebb2, ebb1]
         // ebb0:
         //    x = 1;
         //    br_table x, ebb2, jt
@@ -1039,9 +1039,9 @@ mod tests {
         };
         ssa.def_var(x_var, x1, block0);
 
-        // jt = jump_table ebb2, 0, ebb1
+        // jt = jump_table [ebb2, ebb1]
         jump_table.push_entry(ebb2);
-        jump_table.set_entry(2, ebb1);
+        jump_table.push_entry(ebb1);
         let jt = func.create_jump_table(jump_table);
 
         // ebb0:

lib/frontend/src/switch.rs

@@ -244,7 +244,7 @@ mod tests {
         let func = setup!(0, [0, 1,]);
         assert_eq!(
             func,
-            "    jt0 = jump_table ebb1, ebb2
+            "    jt0 = jump_table [ebb1, ebb2]
 
 ebb0:
     v0 = iconst.i8 0
@@ -280,8 +280,8 @@ ebb3:
         let func = setup!(0, [0, 1, 5, 7, 10, 11, 12,]);
         assert_eq!(
             func,
-            "    jt0 = jump_table ebb1, ebb2
-    jt1 = jump_table ebb5, ebb6, ebb7
+            "    jt0 = jump_table [ebb1, ebb2]
+    jt1 = jump_table [ebb5, ebb6, ebb7]
 
 ebb0:
     v0 = iconst.i8 0