The `FlowVar::toString` predicate is purely a debugging aid, but
unfortunately it has to be `cached` because it's in a `cached` class.
Before this commit, it caused `Expr::toString` to be evaluated in full.
The `automaticVariableAddressEscapes` predicate got join-ordered badly
in its `unaliased_ssa` version. These are the tuple counts on Wireshark,
where one pipeline step is seen to have 716 million tuples:
```
[2019-03-02 11:29:41] (42s) Starting to evaluate predicate AliasAnalysis::automaticVariableAddressEscapes#2#f
[2019-03-02 11:30:06] (67s) Tuple counts:
353419 ~0% {1} r1 = JOIN project#Instruction::VariableAddressInstruction#class#2#ff WITH AliasAnalysis::resultEscapesNonReturn#2#f ON project#Instruction::VariableAddressInstruction#class#2#ff.<0>=AliasAnalysis::resultEscapesNonReturn#2#f.<0> OUTPUT FIELDS {AliasAnalysis::resultEscapesNonReturn#2#f.<0>}
353419 ~0% {2} r2 = JOIN r1 WITH IRConstruction::Cached::getInstructionEnclosingFunctionIR#ff@staged_ext ON r1.<0>=IRConstruction::Cached::getInstructionEnclosingFunctionIR#ff@staged_ext.<0> OUTPUT FIELDS {IRConstruction::Cached::getInstructionEnclosingFunctionIR#ff@staged_ext.<1>,r1.<0>}
353419 ~0% {2} r3 = JOIN r2 WITH FunctionIR::FunctionIR::getFunction_dispred#3#ff ON r2.<0>=FunctionIR::FunctionIR::getFunction_dispred#3#ff.<0> OUTPUT FIELDS {FunctionIR::FunctionIR::getFunction_dispred#3#ff.<1>,r2.<1>}
716040298 ~0% {2} r4 = JOIN r3 WITH IRVariable::IRVariable#class#3#ff_10#join_rhs ON r3.<0>=IRVariable::IRVariable#class#3#ff_10#join_rhs.<0> OUTPUT FIELDS {IRVariable::IRVariable#class#3#ff_10#join_rhs.<1>,r3.<1>}
4480139 ~0% {2} r5 = JOIN r4 WITH IRVariable::IRAutomaticVariable#class#3#ff ON r4.<0>=IRVariable::IRAutomaticVariable#class#3#ff.<0> OUTPUT FIELDS {r4.<1>,r4.<0>}
66760 ~91% {1} r6 = JOIN r5 WITH Instruction::VariableInstruction::getVariable_dispred#2#ff ON r5.<0>=Instruction::VariableInstruction::getVariable_dispred#2#ff.<0> AND r5.<1>=Instruction::VariableInstruction::getVariable_dispred#2#ff.<1> OUTPUT FIELDS {r5.<1>}
return r6
[2019-03-02 11:30:06] (67s) >>> Relation AliasAnalysis::automaticVariableAddressEscapes#2#f: 35531 rows using 0 MB
```
The predicate contained a cyclic join, which is always hard to optimize.
I couldn't see a reason to join the `FunctionIR`, so I removed that
part. The predicate is now fast, and there are no changes in the tests.
This implementation is borrowed from Java's QL library and offers a
great performance improvement. For example, on Wireshark the performance
goes from
Dominance::bbDominates#ff ....... 40.3s
SSAUtils::dominanceFrontier#ff .. 30s
to
SSAUtils::dominanceFrontier#ff .. 418ms (executed 67 times)
The big performance problem before was the need to materialize
`bbDominates`, which is the reflexive-transitive "basic block dominates"
relation. It had 79 million rows on Wireshark.
