Fix unaliasing ExchangeNode outputs

Reintroduces separate mapping strategy for the cases of single source
and multiple sources of ExchangeNode.
The previous separation was lost when refactoring the class,
resulting in potentially creating longer mapping paths.

presto-main/src/main/java/io/prestosql/sql/planner/optimizations/UnaliasSymbolReferences.java

@@ -297,21 +297,22 @@ public PlanAndMappings visitExchange(ExchangeNode node, UnaliasContext context)
                     }
                 }
             }
-
-            // 2. for multiple ExchangeNode sources, if different output symbols result from the same lists of canonical input symbols, map all those outputs to the same symbol
-            Map<List<Symbol>, Symbol> inputsToOutputs = new HashMap<>();
-            for (int i = 0; i < rewrittenOutputs.size(); i++) {
-                ImmutableList.Builder<Symbol> inputsBuilder = ImmutableList.builder();
-                for (List<Symbol> inputs : rewrittenInputs) {
-                    inputsBuilder.add(inputs.get(i));
-                }
-                List<Symbol> inputs = inputsBuilder.build();
-                Symbol previous = inputsToOutputs.get(inputs);
-                if (previous == null || rewrittenOutputs.get(i).equals(previous)) {
-                    inputsToOutputs.put(inputs, rewrittenOutputs.get(i));
-                }
-                else {
-                    newMapping.put(rewrittenOutputs.get(i), previous);
+            else {
+                // 2. for multiple ExchangeNode sources, if different output symbols result from the same lists of canonical input symbols, map all those outputs to the same symbol
+                Map<List<Symbol>, Symbol> inputsToOutputs = new HashMap<>();
+                for (int i = 0; i < rewrittenOutputs.size(); i++) {
+                    ImmutableList.Builder<Symbol> inputsBuilder = ImmutableList.builder();
+                    for (List<Symbol> inputs : rewrittenInputs) {
+                        inputsBuilder.add(inputs.get(i));
+                    }
+                    List<Symbol> inputs = inputsBuilder.build();
+                    Symbol previous = inputsToOutputs.get(inputs);
+                    if (previous == null || rewrittenOutputs.get(i).equals(previous)) {
+                        inputsToOutputs.put(inputs, rewrittenOutputs.get(i));
+                    }
+                    else {
+                        newMapping.put(rewrittenOutputs.get(i), previous);
+                    }
                 }
             }