Heap usage of GHC increased in 7.8 vs. 7.6

[Simon Marlow]

I noticed that T1969 is failing again, and decided to do a little 
investigation.  The maximum residency when compiling T1969 with HEAD 
compared with 7.6.3 looks like this:

7.6.3: 10,473,920
HEAD:  13,783,536

This is with +RTS -h -i0.01 to avoid sampling errors as much as 
possible.  The figures are pretty accurate, and the heap profiles 
confirm it: we're using a lot more heap now.

-ddump-if-trace shows that HEAD is reading more interface files:

Renamer stats: 10 interfaces read
                6 type/class/variable imported, out of 1361 read
                0 instance decls imported, out of 105 read
                0 rule decls imported, out of 53 read

vs. with 7.6.3:

Renamer stats: 8 interfaces read
                2 type/class/variable imported, out of 828 read
                0 instance decls imported, out of 40 read
                0 rule decls imported, out of 45 read

The extra interface files are Control.Applicative and Control.Monad.  So 
the question is, why are we now reading these on *every* compilation? 
(T1969 imports only the Prelude).  Presumably this is something to do 
with the AMP warnings, but can't we lazily load these interfaces when 
they're needed?

Cheers,
Simon