fix: Ensure flat input for functions that consume a single field

Ensure that Velox's guarantee that functions consuming a single field
reference as input always receive a flat input holds, even when the
debug query config disables peeling is turned on.

Test Plan:
Added a unit test

velox/expression/Expr.cpp

@@ -1470,10 +1470,14 @@ bool Expr::applyFunctionWithPeeling(
   LocalDecodedVector localDecoded(context);
   LocalSelectivityVector newRowsHolder(context);
   if (!context.peelingEnabled()) {
-    if (inputValues_.size() == 1) {
+    if (distinctFields_.size() < 2) {
       // If we have a single input, velox needs to ensure that the
-      // vectorFunction would receive a flat input.
-      BaseVector::flattenVector(inputValues_[0]);
+      // vectorFunction would receive a flat or constant input.
+      for (int i = 0; i < inputValues_.size(); ++i) {
+        if (inputValues_[i]->encoding() == VectorEncoding::Simple::DICTIONARY) {
+          BaseVector::flattenVector(inputValues_[i]);
+        }
+      }
       applyFunction(applyRows, context, result);
       return true;
     }

velox/expression/tests/ExprTest.cpp

@@ -4861,6 +4861,15 @@ TEST_F(ExprTest, disablePeeling) {
       makeRowVector({flatInput}),
       {},
       execCtx.get()));
+
+  // Ensure functions that take a single column as input but can have more
+  // constant inputs also receive a flat vector. We use the in-predicate in this
+  // case which has a check for ensuring flat input.
+  ASSERT_NO_THROW(evaluateMultiple(
+      {"dict_wrap(c0) in (40, 42)"},
+      makeRowVector({flatInput}),
+      {},
+      execCtx.get()));
 }
 
 TEST_F(ExprTest, disableSharedSubExpressionReuse) {