"lazy" leads to undefined reference to `stg_ap_0_upd_info'

This is basically the same thing as #11155 (closed) but triggered differently.

-- M.hs
{-# OPTIONS_GHC -O0 #-}
module M(f) where

import GHC.Exts

{-# NOINLINE z #-}
z = ()

f :: () -> ()
f _ = let x = lazy z
      in g x x

{-# NOINLINE g #-}
g :: () -> () -> ()
g _ _ = ()

-- MM.hs
import M
main = f `seq` return ()

On GHC 8.0, I get:

ezyang@sabre:~$ ghc-8.0 --make MM.hs -fforce-recomp
[1 of 2] Compiling M                ( M.hs, M.o )
[2 of 2] Compiling Main             ( MM.hs, MM.o )
Linking MM ...
./M.o: In function `r2aD_info':
(.text+0x4a): undefined reference to `stg_ap_0_upd_info'
collect2: error: ld returned 1 exit status
`gcc' failed in phase `Linker'. (Exit code: 1)

Error goes away when you turn on optimization. My understanding is that removing lazy in CorePrep is too late, because there's no pass afterwards that eliminates the trivial thunk assignment. Here's the STG:

z_r2aC :: ()
[GblId, Caf=NoCafRefs, Str=DmdType, Unf=OtherCon []] =
    NO_CCS ()! [];

g_r2aD :: () -> () -> ()
[GblId, Arity=2, Caf=NoCafRefs, Str=DmdType, Unf=OtherCon []] =
    sat-only \r srt:SRT:[] [ds_s2rW ds1_s2rX] () [];

M.f :: () -> ()
[GblId, Arity=1, Caf=NoCafRefs, Str=DmdType, Unf=OtherCon []] =
    \r srt:SRT:[] [ds_s2rY]
        let {
          x_s2rZ :: ()
          [LclId, Str=DmdType, Unf=OtherCon []] =
              \u srt:SRT:[] [] z_r2aC;
        } in  g_r2aD x_s2rZ x_s2rZ;

so we need, instead, something like x_s2rZ = z_r2aC.

This is blocking D2211, where the insertion of a noinline (which is optimized out similarly to lazy) triggers a stage 2 linker failure when you don't compile with optimization (unfortunately, this is NOT caught validate since we build GHC with optimization... but with this test it will be!)

Perhaps there should be an ASSERT in the codegen so it doesn't create this symbol?