add ExprUsesArgs analysis

[yihozhang]

This analysis returns a set of variables used by an expression. Users of this analysis need to explicitly place demands on the expression they want to run this analysis on.

[oflatt]

Why does this PR require using a set? Not sure what it is used for yet.
Could you do a relation (relation ExprUsesVar (Expr Expr))?

[yihozhang]

There are several reasons for this:

• I need to collect a list FV of all free variables of an expression, and add FV to the input context of let-inlining. Collecting all values of a column in a container is less straightforward than just having a set as output.
• There was a typo in the original analysis. The merge function for ExprUsesArgs should be set-intersect, otherwise a constant may use variables if it is merged with x-x, etc. relation cannot do this.

[oflatt]

So this is more like a ExprAlwaysUsesArgs analysis? As in, it uses these arguments in all equivalent versions?

[oflatt]

I think we decided to close this, since we are changing it to extract a particular term that uses a particular set of arguments.