protocol redundancy remover

[rolph-recto]

There are certain situations where redundant PDG nodes are selected protocols, which makes selection slower.

Scenario 1: redundant queries

The following generates two queries to shell, manifest in the ANF translation:

if (1 <= shell && shell <= 3) { ... }

ANF translation:

let tmp1 = shell.get()
let tmp2 = shell.get()
let tmp3 = 1 <= tmp1
let tmp4 = tmp2 <= 3
let tmp5 = tmp3 && tmp4
if (tmp5) { ... }

but since it's impossible for shell to be modified in the middle of evaluating the expression, this can be a single query. Like so:

let tmp1 = shell.get()
let tmp3 = 1 <= tmp1
let tmp4 = tmp1 <= 3
let tmp5 = tmp3 && tmp4
if (tmp5) { ... }

Scenario 2: downgrades

Downgrades are useless after label inference, so they can be removed.

Example:

let tmp1 = x
let tmp2 = downgrade(x, {...})
if (tmp2) { ... }

Can be rewritten as:

let tmp1 = x
if (tmp1) { ... }

The solution is to have a redundancy removal pass before generating the PDG.