C++: Performance tweak for 1-field struct loads

On kamailio/kamailio the `DataFlowUtil::simpleInstructionLocalFlowStep`
predicate was slow because of the case for single-field structs, where
there was a large tuple-count bulge when joining with
`getFieldSizeOfClass`:

    3552902   ~2%       {2} r1 = SCAN Instruction::CopyInstruction::getSourceValueOperand_dispred#3#ff AS I OUTPUT I.<1>, I.<0>
    2065347   ~2%       {2} r35 = JOIN r1 WITH Operand::NonPhiMemoryOperand::getAnyDef_dispred#3#ff AS R ON FIRST 1 OUTPUT r1.<1>, R.<1>
    2065827   ~2%       {3} r36 = JOIN r35 WITH Instruction::Instruction::getResultType_dispred#3#ff AS R ON FIRST 1 OUTPUT R.<1>, r35.<1>, r35.<0>
    2065825   ~3%       {3} r37 = JOIN r36 WITH Type::Type::getSize_dispred#ff AS R ON FIRST 1 OUTPUT r36.<1>, r36.<2>, R.<1>
    2068334   ~2%       {4} r38 = JOIN r37 WITH Instruction::Instruction::getResultType_dispred#3#ff AS R ON FIRST 1 OUTPUT R.<1>, r37.<2>, r37.<0>, r37.<1>
    314603817 ~0%       {3} r39 = JOIN r38 WITH DataFlowUtil::getFieldSizeOfClass#fff_120#join_rhs AS R ON FIRST 2 OUTPUT r38.<3>, R.<2>, r38.<2>
    8         ~0%       {2} r40 = JOIN r39 WITH Instruction::Instruction::getResultType_dispred#3#ff AS R ON FIRST 2 OUTPUT r39.<2>, r39.<0>

That's 314M tuples.

Strangely, there is no such bulge on more well-behaved snapshots like
mysql/mysql-server.

With this commit the explosion is gone:

    ...
    2065825  ~0%       {4} r37 = JOIN r36 WITH Type::Type::getSize_dispred#ff AS R ON FIRST 1 OUTPUT r36.<0>, R.<1>, r36.<1>, r36.<2>
    1521     ~1%       {3} r38 = JOIN r37 WITH DataFlowUtil::getFieldSizeOfClass#fff_021#join_rhs AS R ON FIRST 2 OUTPUT r37.<2>, R.<2>, r37.<3>
    8        ~0%       {2} r39 = JOIN r38 WITH Instruction::Instruction::getResultType_dispred#3#ff AS R ON FIRST 2 OUTPUT r38.<0>, r38.<2>

Author: jbj

cpp/ql/src/semmle/code/cpp/ir/dataflow/internal/DataFlowUtil.qll

@@ -564,10 +564,11 @@ private predicate simpleInstructionLocalFlowStep(Instruction iFrom, Instruction
   or
   // Flow from stores to structs with a single field to a load of that field.
   iTo.(LoadInstruction).getSourceValueOperand().getAnyDef() = iFrom and
-  exists(int size, Type type |
+  exists(int size, Type type, Class cTo |
     type = iFrom.getResultType() and
-    iTo.getResultType().getSize() = size and
-    getFieldSizeOfClass(iTo.getResultType(), type, size)
+    cTo = iTo.getResultType() and
+    cTo.getSize() = size and
+    getFieldSizeOfClass(cTo, type, size)
   )
   or
   // Flow through modeled functions