Missing constant folding in STG

Summary

This is a followup of #25166 (closed): we don't generate static closures in some cases. This was found while investigating #25649

Steps to reproduce

Compile the following code with -O2 -ddump-cmm:

module M where

import Data.Word

data Alice = Alice1 {-# UNPACK #-} !Word32 | Alice2 {-# UNPACK #-} !Bob
data Bob = Bob1 | Bob2

foo = Alice1 10
bar = Alice2 Bob2

data Carlos = Carlos {-# UNPACK #-} !Alice

baz_foo = Carlos foo
baz_bar = Carlos bar        

In the Cmm output, we see that baz_bar and baz_foo are dynamically allocated:

[M.baz_bar_entry() { //  [R1]
...
       cLw: // global
           I64[Sp - 16] = stg_bh_upd_frame_info;
           I64[Sp - 8] = _cLv::I64;
           I64[Hp - 16] = M.Carlos_con_info;
           I64[Hp - 8] = 2;
           I64[Hp] = 2;
           R1 = Hp - 15;
           Sp = Sp - 16;
           call (P64[Sp])(R1) args: 24, res: 0, upd: 24;
     }
 },
 section ""data" . M.baz_bar_closure" {
     M.baz_bar_closure:
         const M.baz_bar_info;
         const 0;
         const 0;
         const 0;
 }]

Expected behavior

I would expect them to be statically allocated. I.e. for baz_bar above:

 section ""data" . M.baz_bar_closure" {
     M.baz_bar_closure:
         const M.Carlos_con_info;
         const 2;
         const 2;
 }]

Analysis

If we -ddump-stg-final, we can see that these closures are dynamically allocated because constant folding isn't performed in STG:

M.baz_foo :: M.Carlos
[GblId, Unf=OtherCon []] =
    {} \u []
        case word32ToWord# [10#Word32] of cst_sum_gJU {
        __DEFAULT -> M.Carlos [1# cst_sum_gJU];
        };

M.baz_bar :: M.Carlos
[GblId, Unf=OtherCon []] =
    {} \u []
        case int2Word# [2#] of cst_sum_gJZ {
        __DEFAULT -> M.Carlos [2# cst_sum_gJZ];
        };

We should improve the STG optimizer to perform them.

Environment

• GHC version used:

Optional:

• Operating System:
• System Architecture: