C++: Replace "min = max" with "unique"

With the new `unique` aggregate added to QL, we can express directly
what the "min = max" pattern emulates.

Replacing "min and max" with `unique` might in general lead to fewer
results, but that happens only in cases where the aggregate expression
has multiple values. For the three predicates changed in this commit,
that should only happen on malformed databases.

cpp/ql/src/semmle/code/cpp/commons/Buffer.qll

@@ -92,13 +92,7 @@ int getBufferSize(Expr bufferExpr, Element why) {
     // dataflow (all sources must be the same size)
     bufferExprNode = DataFlow::exprNode(bufferExpr) and
     result =
-      min(Expr def |
-        DataFlow::localFlowStep(DataFlow::exprNode(def), bufferExprNode)
-      |
-        getBufferSize(def, _)
-      ) and
-    result =
-      max(Expr def |
+      unique(Expr def |
         DataFlow::localFlowStep(DataFlow::exprNode(def), bufferExprNode)
       |
         getBufferSize(def, _)

cpp/ql/src/semmle/code/cpp/controlflow/internal/ConstantExprs.qll

@@ -532,13 +532,7 @@ library class ExprEvaluator extends int {
       interestingVariableAccess(e, va, v, true) and
       // All assignments must have the same int value
       result =
-        min(Expr value |
-          value = v.getAnAssignedValue() and not ignoreVariableAssignment(e, v, value)
-        |
-          getValueInternalNonSubExpr(value)
-        ) and
-      result =
-        max(Expr value |
+        unique(Expr value |
           value = v.getAnAssignedValue() and not ignoreVariableAssignment(e, v, value)
         |
           getValueInternalNonSubExpr(value)

cpp/ql/src/semmle/code/cpp/ir/implementation/aliased_ssa/constant/ConstantAnalysis.qll

@@ -14,8 +14,7 @@ int getConstantValue(Instruction instr) {
   or
   exists(PhiInstruction phi |
     phi = instr and
-    result = max(Operand op | op = phi.getAnInputOperand() | getConstantValue(op.getDef())) and
-    result = min(Operand op | op = phi.getAnInputOperand() | getConstantValue(op.getDef()))
+    result = unique(Operand op | op = phi.getAnInputOperand() | getConstantValue(op.getDef()))
   )
 }
 

cpp/ql/src/semmle/code/cpp/ir/implementation/raw/constant/ConstantAnalysis.qll

@@ -14,8 +14,7 @@ int getConstantValue(Instruction instr) {
   or
   exists(PhiInstruction phi |
     phi = instr and
-    result = max(Operand op | op = phi.getAnInputOperand() | getConstantValue(op.getDef())) and
-    result = min(Operand op | op = phi.getAnInputOperand() | getConstantValue(op.getDef()))
+    result = unique(Operand op | op = phi.getAnInputOperand() | getConstantValue(op.getDef()))
   )
 }
 

cpp/ql/src/semmle/code/cpp/ir/implementation/unaliased_ssa/constant/ConstantAnalysis.qll

@@ -14,8 +14,7 @@ int getConstantValue(Instruction instr) {
   or
   exists(PhiInstruction phi |
     phi = instr and
-    result = max(Operand op | op = phi.getAnInputOperand() | getConstantValue(op.getDef())) and
-    result = min(Operand op | op = phi.getAnInputOperand() | getConstantValue(op.getDef()))
+    result = unique(Operand op | op = phi.getAnInputOperand() | getConstantValue(op.getDef()))
   )
 }