Recursive let-expressions throw DEC transformation into a loop

[christiaanb]

The following reduced test-case:

import Clash.Prelude

incr :: Index 2 -> Index 2
incr i = if i == maxBound then 0 else i + 1
{-# NOINLINE incr #-}

topEntity :: Index 10 -> Index 2
topEntity j = case j < 1 of
  False ->
    let xs = init (Cons 1 (Cons (incr (head xs)) Nil)) in last xs
  True ->
    let ys = init (Cons 2 (Cons (incr (last ys)) Nil)) in head ys

throws the DEC transformation into a loop. Specifically, DEC rewrites:

case j < 1 of
  False ->
    let xs = init (Cons 1 (Cons (incr (head xs)) Nil)) in last xs
  True ->
    let ys = init (Cons 2 (Cons (incr (last ys)) Nil)) in head ys

to

let incrOut = incr (case j < 1 of
  False ->
    let xs = init (Cons 1 (Cons (incr (head xs)) Nil)) in head xs
  True ->
    let ys = init (Cons 2 (Cons (incr (last ys)) Nil)) in last ys)
in case j < 1 of
  False ->
    let xs = init (Cons 1 (Cons incrOut Nil)) in last xs
  True ->
    let ys = init (Cons 2 (Cons incrOut Nil)) in head ys

trying to share uses of incr between the two case-alternatives; only to create a new case-expression on which DEC can match again. What DEC should be doing is rewrite to:

let incrOut = incr (case j < 1 of
  False ->
    let xs = init (Cons 1 (Cons incrOut Nil)) in head xs
  True ->
    let ys = init (Cons 2 (Cons incrOut Nil)) in last ys)
in case j < 1 of
  False ->
    let xs = init (Cons 1 (Cons incrOut Nil)) in last xs
  True ->
    let ys = init (Cons 2 (Cons incrOut Nil)) in head ys