[Unity][Transform] Handle replacement at both var binding and usage (#16367)

Resolve a bug that caused undefined relax variables in the output of
`CanonicalizeBindings` for cases where `VisitVarDef(const Var&)`
replaces a variable, and `VisitExpr_(const VarNode*)` returns a value
with different struct info, both occurring within the same
`VarBinding`.

The ExprMutator is only allowed to update a variable's struct info
if the value bound to it has new struct info.  When
CanonicalizeBindings replaces a trivial binding, this may provide
better struct info as a result.

Prior to this commit, `ExprMutator::ReEmitBinding` defined a
remap for `binding->var->vid`, even if the derived class defined a
replacement by overriding `VisitVarDef`.  If the derived class
defines a new variable binding by overriding `VisitVarDef`, and
also causes a variable replacement by overriding `VisitExpr` and
returning a type with different struct info, then `ExprMutator`
must check for both `binding->var->vid` *AND* `new_var->vid`.  The
former may be present in the unmodified graph, and the latter may
be produced by the derived class before delegating to the base
class.

This commit updates `ExprMutator::ReEmitBinding` to define entries for
both replacements that may be required.

Author: Lunderberg

src/relax/ir/expr_functor.cc

@@ -682,7 +682,9 @@ void ExprMutator::ReEmitBinding(const VarBindingNode* binding, Expr new_value) {
   if (!temp.same_as(new_var)) {
     new_var = temp;
   }
+
   this->var_remap_[binding->var->vid] = new_var;
+  this->var_remap_[new_var->vid] = new_var;
 
   builder_->EmitNormalized(VarBinding(new_var, new_value));
 }