Rework br_table to use BlockCall (#5731)

Rework br_table to use BlockCall, allowing us to avoid adding new nodes during ssa construction to hold block arguments. Additionally, many places where we previously matched on InstructionData to extract branch destinations can be replaced with a use of branch_destination or branch_destination_mut.

cranelift/codegen/src/machinst/lower.rs

@@ -942,29 +942,13 @@ impl<'func, I: VCodeInst> Lower<'func, I> {
             // Avoid immutable borrow by explicitly indexing.
             let (inst, succ) = self.vcode.block_order().succ_indices(block)[succ_idx];
 
-            // Get branch args and convert to Regs.
-            let branch_args = match &self.f.dfg.insts[inst] {
-                InstructionData::Jump {
-                    destination: block, ..
-                } => block.args_slice(&self.f.dfg.value_lists),
-                InstructionData::Brif {
-                    blocks: [then_block, else_block],
-                    ..
-                } => {
-                    // NOTE: `succ_idx == 0` implying that we're traversing the `then_block` is
-                    // enforced by the traversal order defined in `visit_block_succs`. Eventually
-                    // we should traverse the `branch_destination` slice there, which would
-                    // simplify computing the branch args significantly.
-                    if succ_idx == 0 {
-                        then_block.args_slice(&self.f.dfg.value_lists)
-                    } else {
-                        assert!(succ_idx == 1);
-                        else_block.args_slice(&self.f.dfg.value_lists)
-                    }
-                }
-                InstructionData::BranchTable { .. } => &[],
-                _ => unreachable!(),
-            };
+            // The use of `succ_idx` to index `branch_destination` is valid on the assumption that
+            // the traversal order defined in `visit_block_succs` mirrors the order returned by
+            // `branch_destination`. If that assumption is violated, the branch targets returned
+            // here will not match the clif.
+            let branches = self.f.dfg.insts[inst].branch_destination(&self.f.dfg.jump_tables);
+            let branch_args = branches[succ_idx].args_slice(&self.f.dfg.value_lists);
+
             let mut branch_arg_vregs: SmallVec<[Reg; 16]> = smallvec![];
             for &arg in branch_args {
                 let arg = self.f.dfg.resolve_aliases(arg);