New codegen: CmmStackLayout igraph memory explosion

Filing this as a ticket because I don't have time to investigate. On compiler performance test T3294, with the new code generator, we see a dramatic spike in allocations due to the igraph function. Looks like the algorithm doesn't like data types with lots and lots of constructors!

Here is the textual profile:

	Tue Apr 26 17:55 2011 Time and Allocation Profiling Report  (Final)

	   ghc-stage2 +RTS -p -RTS -B/home/ezyang/Dev/ghc-build-master-stage2-prof/inplace/lib -pgmc /usr/bin/gcc -pgma /usr/bin/gcc -pgml /usr/bin/gcc -pgmP /usr/bin/gcc -E -undef -traditional -fforce-recomp -dcore-lint -dcmm-lint -dno-debug-output -no-user-package-conf -rtsopts -c T3294.hs

	total time  =       34.74 secs   (1737 ticks @ 20 ms)
	total alloc = 7,892,000,312 bytes  (excludes profiling overheads)

COST CENTRE             MODULE               %time %alloc

addToUFM                UniqFM                 8.0   14.9
dataflowPassFwd         Cmm                    7.2   10.0
foldUFM_Directly        UniqFM                 5.5    7.4
slotLattice             CmmStackLayout         5.2    4.4
conflictSlots           CmmStackLayout         4.0    7.1
igraph                  CmmStackLayout         3.8    2.4
ufmToList               UniqFM                 3.4    7.8
dataflowPassBwd         Cmm                    3.4    4.6
sLit                    FastString             1.8    0.1
layLeft                 Pretty                 1.8    1.5
MAIN                    MAIN                   1.7    0.0
dopt                    DynFlags               1.7    0.0
mkUniqueGrimily         Unique                 1.7    1.3
textBeside_             Pretty                 1.3    1.9
lattice                 CmmProcPoint           1.3    0.1
areaBuilder             CmmStackLayout         1.3    0.9
bPutStr                 BufWrite               1.3    0.4
allocRegsAndSpill_spill RegAlloc.Linear.Main   1.3    0.4
countUses               CmmOpt                 1.3    0.6
genRaInsn               RegAlloc.Linear.Main   1.0    1.0
lookForInline'          CmmOpt                 1.0    0.3
iBox                    FastTypes              1.0    2.6
writeFastMutInt         FastMutInt             0.7    1.6
readFastMutInt          FastMutInt             0.7    1.7
aboveNest               Pretty                 0.7    1.4
<>                      Pretty                 0.6    1.1
foldRightWithKey        FiniteMap              0.6    1.1

And I will attach a more informative hp2ps heap profile.

Trac metadata

Trac field	Value
Version	7.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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Architecture