Simple optimizations for the core rewriter (#3569)

In some SyGuS applications, the bottleneck is the rewriter.  This PR makes a few simple optimizations to the core rewriter, namely:
(1) Minimize the overhead of `theoryOf` calls, which take about 10-15% of the runtime in the rewriter.  This PR avoids many calls to `theoryOf` by the observation that nodes with zero children never rewrite, hence we can return the node itself immediately.  Furthermore, the `theoryOf` call can be simplified to hardcode the context under which we were using it: get the theory (based on type) where due to the above, we can assume that the node is not a variable.
The one (negligible) change in behavior due to this change is that nodes with more than one child for which `isConst` returns true (this is limited to the case of `APPLY_CONSTRUCTOR` in datatypes) lookup their theory based on the Kind, not their type, which should always be the same theory unless a theory had a way of constructing constant nodes of a type belonging to another theory.
(2) Remove deprecated infrastrastructure for a "neverIsConst" function, which was adding about a 1% of the runtime for some SyGuS benchmarks.

This makes SyGuS for some sets of benchmarks roughly 3% faster.

1 files changed, 0 insertions, 4 deletions

diff --git a/src/expr/node.cpp b/src/expr/node.cpp
index de1d5475b..a8fc9d54c 100644
--- a/src/expr/node.cpp
+++ b/src/expr/node.cpp
@@ -91,10 +91,6 @@ bool NodeTemplate<ref_count>::isConst() const {
     Debug("isConst") << "Node::isConst() returning false, it's a VARIABLE" << std::endl;
     return false;
   default:
-    if(expr::TypeChecker::neverIsConst(NodeManager::currentNM(), *this)){
-      Debug("isConst") << "Node::isConst() returning false, the kind is never const" << std::endl;
-      return false;
-    }
     if(getAttribute(IsConstComputedAttr())) {
       bool bval = getAttribute(IsConstAttr());
       Debug("isConst") << "Node::isConst() returning cached value " << (bval ? "true" : "false") << " for: " << *this << std::endl;