[NFC] Convert LocalGraph influences accesses to function calls

[kripken]

This replaces direct access of the data structure graph.*influences[foo] with a call
graph.get*influences(foo). This will allow a later PR to make those calls optionally
lazy.

[tlively]

Suggested change

	// read from it; get influences are the sets that can (directly) read from it.
	// read from it; get influences are the sets from which the get can (directly) read.

[kripken]

IIUC what you wrote, that is actually getSets, not getInfluences. Example:

x = 5;       // line 0
foo(x);      // line 1
y = x + 20;  // line 2

• getSets of the x on line 1 is the set of x on line 0.
• setInfluences of the set on line 0 are the gets of x on lines 1 and 2.
• getInfluences of the x on line 2 is the set of y on line 2.

getSets is the basic property of "what value is being read here?" We go backwards to find the right sets.

*Influences are used for forward propagation (hence the comment says "useful for algorithms that propagate changes"). Imagine that a set is given a new value, then setInfluences is the gets that read it. And, from each of those gets, we want to know which other sets may be influenced by it in turn, to keep propagating forward.

[tlively]

I see. I was confused by the explanation that sets were doing reads and thought that was surely a typo. Perhaps instead of talking about sets doing reads, we can describe the sets' values as using the gets.

[kripken]

Perhaps. Maybe "propagates" can also make more sense rather than "influences", as influence isn't as clearly directional. I'm not sure what's best here, but let's keep thinking about it.