egraphs: a few miscellaneous compile-time optimizations. (#5072)

* egraphs: a few miscellaneous compile-time optimizations.

These optimizations together are worth about a 2% compile-time
reduction, as measured on one core with spidermonkey.wasm as an input,
using `hyperfine` on `wasmtime compile`.

The changes included are:
- Some better pre-allocation (blockparams and side-effects concatenated
  list vecs);
- Avoiding the indirection of storing list-of-types for every Pure and
  Inst node, when almost all nodes produce only a single result;
  instead, store arity and single type if it exists, and allow result
  projection nodes to fill in types otherwise;
- Pack the `MemoryState` enum into one `u32` (this together with the
  above removal of the type slice allows `Node` to
  shrink from 48 bytes to 32 bytes);
- always-inline an accessor (`entry` on `CtxHash`) that wasn't
  (`always(inline)` appears to be load-bearing, rather than just
  `inline`);
- Split the update-analysis path into two hotpaths, one for the union
  case and one for the new-node case (and the former can avoid
  recomputing for the contained node when replacing a node with
  node-and-child eclass entry).

* Review feedback.

* Fix test build.

* Fix to lowering when unused output with invalid type is present.

cranelift/codegen/src/ir/dfg.rs

@@ -274,6 +274,12 @@ impl DataFlowGraph {
         self.values[v].ty()
     }
 
+    /// Fill in the type of a value, only if currently invalid (as a placeholder).
+    pub(crate) fn fill_in_value_type(&mut self, v: Value, ty: Type) {
+        debug_assert!(self.values[v].ty().is_invalid());
+        self.values[v].set_type(ty);
+    }
+
     /// Get the definition of a value.
     ///
     /// This is either the instruction that defined it or the Block that has the value as an