Performance bug maybe

Hi,

I think I found a performance bug. Either that, or something is going
on that I don't understand.

I've attached an example program. Here's my session with it:

===============
ghc --make -O2 MaybeBug.hs; time ./MaybeBug +RTS -tstderr

[1 of 1] Compiling Main             ( MaybeBug.hs, MaybeBug.o )
Linking MaybeBug ...
./MaybeBug +RTS -tstderr
-1452071552
191905792
<<ghc: 800037360 bytes, 4 GCs, 40960/40960 avg/max bytes residency (1
samples), 383M in use, 0.00 INIT (0.00 elapsed), 3.96 MUT (4.36
elapsed), 0.00 GC (0.00 elapsed) :ghc>>

real	0m4.408s
user	0m3.960s
sys	0m0.448s

ghc --make -O2 MaybeBug.hs -DSLOW -no-recomp; time ./MaybeBug +RTS -tstderr
[1 of 1] Compiling Main             ( MaybeBug.hs, MaybeBug.o )
Linking MaybeBug ...
./MaybeBug +RTS -tstderr
-1452071552
191905792
<<ghc: 4000037408 bytes, 6106 GCs, 400588800/400588800 avg/max bytes