[llvm] Introduce CounterExpressionBuilder::subst(C, Map) (PR #112698)

[NAKAMURA Takumi via llvm-commits]

================
@@ -135,6 +135,31 @@ Counter CounterExpressionBuilder::subtract(Counter LHS, Counter RHS,
   return Simplify ? simplify(Cnt) : Cnt;
 }
 
+Counter CounterExpressionBuilder::subst(Counter C, const SubstMap &Map) {
+  // Replace C with the value found in Map even if C is Expression.
+  if (auto I = Map.find(C); I != Map.end())
+    return I->second;
+
+  if (!C.isExpression())
+    return C;
+
+  auto CE = Expressions[C.getExpressionID()];
+  auto NewLHS = subst(CE.LHS, Map);
+  auto NewRHS = subst(CE.RHS, Map);
+
+  // Reconstruct Expression with induced subexpressions.
+  switch (CE.Kind) {
----------------
chapuni wrote:

This `switch` should catch them since this doesn't have `default:`.

https://github.com/llvm/llvm-project/pull/112698