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/remove_constant_phis.rs

@@ -7,7 +7,6 @@ use crate::ir;
 use crate::ir::Function;
 use crate::ir::{Block, BlockCall, Inst, Value};
 use crate::timing;
-use arrayvec::ArrayVec;
 use bumpalo::Bump;
 use cranelift_entity::SecondaryMap;
 use smallvec::SmallVec;
@@ -171,9 +170,10 @@ struct BlockSummary<'a> {
     /// We don't bother to include transfers that pass zero parameters
     /// since that makes more work for the solver for no purpose.
     ///
-    /// Note that, because blocks used with `br_table`s cannot have block
-    /// arguments, there are at most two outgoing edges from these blocks.
-    dests: ArrayVec<OutEdge<'a>, 2>,
+    /// We optimize for the case where a branch instruction has up to two
+    /// outgoing edges, as unconditional jumps and conditional branches are
+    /// more prominent than br_table.
+    dests: SmallVec<[OutEdge<'a>; 2]>,
 }
 
 impl<'a> BlockSummary<'a> {
@@ -239,7 +239,11 @@ pub fn do_remove_constant_phis(func: &mut Function, domtree: &mut DominatorTree)
         let mut summary = BlockSummary::new(&bump, formals);
 
         for inst in func.layout.block_insts(b) {
-            for (ix, dest) in func.dfg.insts[inst].branch_destination().iter().enumerate() {
+            for (ix, dest) in func.dfg.insts[inst]
+                .branch_destination(&func.dfg.jump_tables)
+                .iter()
+                .enumerate()
+            {
                 if let Some(edge) = OutEdge::new(&bump, &func.dfg, inst, ix, *dest) {
                     summary.dests.push(edge);
                 }
@@ -382,8 +386,8 @@ pub fn do_remove_constant_phis(func: &mut Function, domtree: &mut DominatorTree)
             }
 
             let dfg = &mut func.dfg;
-            let block =
-                &mut dfg.insts[edge.inst].branch_destination_mut()[edge.branch_index as usize];
+            let dests = dfg.insts[edge.inst].branch_destination_mut(&mut dfg.jump_tables);
+            let block = &mut dests[edge.branch_index as usize];
 
             old_actuals.extend(block.args_slice(&dfg.value_lists));