The predicate
```
argumentValueFlowsThrough(ArgumentNode arg, OutNode out, CallContext cc)
```
has been generalized to
```
argumentValueFlowsThrough(
DataFlowCall call, ArgumentNode arg, Node out, ContentOption contentIn,
ContentOption contentOut
)
```
This enables us to summarize normal flow-through (as before), getters, setters,
as well as getter-setters.
We were hitting a combinatorial explosion in `hasDefinitionAtRank` for functions that contain a large number of string literals. The problem was that every `Chi` instruction for `AliasedVirtualVariable` was treated as a definition of every string literal. We already mark string literals as `isReadOnly()`, but we were allowing `AliasedVirtualVariable` to define read-only locations so that the `AliasedDefinition` instruction would provide the initial definition for all string literals.
To fix this, I've introduced the new `InitializeNonLocal` instruction, which is inserted in the prologue of every function right after `AliasedDefinition`. It provides the initial definition for every non-stack memory location, including read-only locations, but is never written to anywhere else. It is the conterpart of the `AliasedUse` instruction in the function epilogue, which represents the use of all non-stack memory after the function returns. I considered renaming `AliasedUse` to `ReturnNonLocal`, to match the `InitializeXXX`/`ReturnXXX` pattern we already use for parameters and indirections, but held off to avoid unnecessary churn. Any thoughts on whether I should make this name change?
This change has a significant speedup in evaluation time for a few of our troublesome databases:
`attnam/ivan`: 13%
`awslabs/s2n`: 26%
`SinaMostafanejad/OpenRDM`: 7%
`zcoinofficial/zcoin`: 8%
Instead of tracking `IRVariable`s directly, alias analysis now tracks instances of the `Allocation` type provided by its `Configuration` parameter. For unaliased SSA, an `Allocation` is just an `IRAutomaticVariable`. For aliased SSA, an `Allocation` is either an `IRVariable` or the memory pointed to by an indirect parameter.
There was already a `WriteSideEffectInstruction` class that served as a
superclass for all the specific write side effects. This new class
serves the same purpose for read side effects.
