[NFC] Refactor Heap2Local logic (#6473)

Separate out an EscapeAnalyzer class that does the escape analysis,
and a Struct2Local one that does the optimization.

Also make a few things const here to be safer.

src/ir/branch-utils.h

@@ -419,10 +419,14 @@ struct BranchTargets {
 
   // Gets the expression that defines this branch target, i.e., where we branch
   // to if we branch to that name.
-  Expression* getTarget(Name name) { return inner.targets[name]; }
+  Expression* getTarget(Name name) const {
+    auto iter = inner.targets.find(name);
+    assert(iter != inner.targets.end());
+    return iter->second;
+  }
 
   // Gets the expressions branching to a target.
-  std::unordered_set<Expression*> getBranches(Name name) {
+  std::unordered_set<Expression*> getBranches(Name name) const {
     auto iter = inner.branches.find(name);
     if (iter != inner.branches.end()) {
       return iter->second;

src/ir/parents.h

@@ -24,7 +24,13 @@ namespace wasm {
 struct Parents {
   Parents(Expression* expr) { inner.walk(expr); }
 
-  Expression* getParent(Expression* curr) { return inner.parentMap[curr]; }
+  Expression* getParent(Expression* curr) const {
+    auto iter = inner.parentMap.find(curr);
+    if (iter != inner.parentMap.end()) {
+      return iter->second;
+    }
+    return nullptr;
+  }
 
 private:
   struct Inner