Bignum literals aren't always considered as constructor applications

Reproducer:

module Bug where

import GHC.Num.Integer

foo :: Int
foo = case 2 of
  IS _ -> 9999
  IP _ -> 0
  IN _ -> 0

Even with -O2 with HEAD we get the following Core:

foo2 = I# 9999#
foo1 = I# 0#
foo3 = 2
foo
  = case foo3 of {
      IS ds_dxZ -> foo2;
      IP ds_dy0 -> foo1;
      IN ds_dy1 -> foo1
    }

while we would expect:

foo = I# 9999#

It's because bignum literals are only desugared into IS/IN/IP constructor applications in Core prep; after Core optimisations. In some places (e.g. rules) we are careful to match on both bignum literal representations but usually to extract a Literal from a ConApp (e.g. IS 2# ==> 2). We would have to do the same in the opposite direction but exprIsConApp_maybe currently doesn't handle bignum literals.

I've found this while working on #15327 (closed). Luckily I already have a branch that fixes this because having two representations in Core for bignum literals already bothered me. Instead of adding another special case to exprIsConApp_maybe, it removes Integer and Natural literals to only keep BigNat# literals.