Investigate compile-time regressions in regex-tdfa-1.2.2

hvr points out that GHC 8.2 takes significantly longer to compile regex-tdfa-1.2.2 than 8.0.2. Interestingly, it seems that compiler allocations fell dramatically while compilation time rose.

GHC 8.0.2:

<<ghc: 89544610040 bytes, 2301 GCs, 189183965/391103872 avg/max bytes
residency (29 samples), 1075M in use, 0.001 INIT (0.001 elapsed),
55.747 MUT (61.520 elapsed), 23.276 GC (23.278 elapsed) :ghc>>

GHC 8.2.1:

<<ghc: 115210990008 bytes, 6383 GCs, 116487828/234031864 avg/max bytes
residency (77 samples), 670M in use, 0.001 INIT (0.000 elapsed),
64.154 MUT (68.262 elapsed), 37.114 GC (37.077 elapsed) :ghc>>

Reproducing

$ cabal unpack regex-tdfa-1.2.2
$ cd regex-tdfa-1.2.2
$ cabal install --only-dependencies
$ cabal configure
$ cabal build
$ time ghc -idist/build/autogen -O2 Text/Regex/TDFA.hs \
   -XMagicHash -XFlexibleInstances -XMultiParamTypeClasses \
   -XRecursiveDo -XBangPatterns -XRankNTypes -XUnboxedTuples \
   -XFlexibleContexts -XUnliftedFFITypes

Trac metadata

Trac field	Value
Version	8.0.1
Type	Task
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler
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