[compiler] simplify PruneDeadFields

[patrick-schultz]

Makes some further progress on simplifying the PruneDeadFields pass, with the primary goal of decoupling it from the details of the binding structure.

The primary change is to memoizeValueIR. Before, it passed in only the requested type of the node, and returned and environment containing all free variables and their requested types. Any bound variables would then need to be removed, and the environments of all children then merged. This low-level manipulation of environments made it closely tied to the binding structure, essentially redundantly encoding everything in Binds.scala.

Now we pass an environment down into the children, which maps variables to a mutable state tracking the requested type. Each Ref node unions the requested type at the reference with the state in the environment. This lets us use the general environment infrastructure.

I didn't do an assertion directly comparing the old and new implementations, as I've done with some other pass rewrites. But PruneDeadFields has pretty good test coverage.

[patrick-schultz]

• [compiler] lift relational lets first in lowering pipeline #14533
• [compiler] simplify PruneDeadFields #14509 👈
• [compiler] simplify NormalizeNames #14514 : 1 other dependent PR (#14554 )
• [compiler] refactor types in bindings #14517
• main

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[ehigham]

rebase plz

[ehigham]

brilliant