GHC 7.10 generates slower code than older versions

I realized that the latest version of GHC (7.10.3) produces significantly slower code than an older version. My current version as of now:

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 7.10.3

I have also two other old versions installed on my local machine.

My test code is taken from here (the collatz1.hs code):

import Data.Word
import Data.List
import System.Environment

collatzNext :: Word32 -> Word32
collatzNext a = (if even a then a else 3*a+1) `div` 2

-- new code
collatzLen :: Word32 -> Int
collatzLen a0 = lenIterWhile collatzNext (/= 1) a0

lenIterWhile :: (a -> a) -> (a -> Bool) -> a -> Int
lenIterWhile next notDone start = len start 0 where
    len n m = if notDone n
                then len (next n) (m+1)
                else m
-- End of new code

main = do
    [a0] <- getArgs
    let max_a0 = (read a0)::Word32
    print $ maximum $ map (\a0 -> (collatzLen a0, a0)) [1..max_a0]

Compiling with GHC 7.4, 7.6 and 7.10 yields the following times:

$ ~/Tools/ghc-7.4.2/bin/ghc -O2 Test.hs 
[1 of 1] Compiling Main             ( Test.hs, Test.o )
Linking Test ...

$ time ./Test 1000000
(329,837799)

real    0m1.879s
user    0m1.876s
sys     0m0.000s

$ ~/Tools/ghc-7.6.1/bin/ghc -O2 Test.hs 
[1 of 1] Compiling Main             ( Test.hs, Test.o )
Linking Test ...

$ time ./Test 1000000
(329,837799)

real    0m1.901s
user    0m1.896s
sys     0m0.000s

$ ~/Tools/ghc/bin/ghc -O2 Test.hs 
[1 of 1] Compiling Main             ( Test.hs, Test.o )
Linking Test ...

$ time ./Test 1000000
(329,837799)

real    0m10.562s
user    0m10.528s
sys     0m0.036s    

We can tell there is no doubt that the latest version of GHC produces worse code than the older two versions. I can't reproduce the same efficiency as the blog though probably because I don't have LLVM and Idon't have the exact version the author used. But still, I believe the conclusion is obvious.

My question is, in general, why this could happen? Somehow GHC becomes worse than it used to be. And specifically, if I want to investigate, how should I get myself started?