Improve SpecConstr for local bindings: seed specialisation from the calls
This patch makes a significant improvement to SpecConstr, based on
Roman's experience with using it for stream fusion. The main change is
this:
* For local (not-top-level) declarations, seed the specialisation
loop from the calls in the body of the 'let'.
See Note [Local recursive groups] for discussion and example. Top-level
declarations are treated just as before.
Other changes in this patch:
* New flag -fspec-constr-count=N sets the maximum number of specialisations
for any single function to N. -fno-spec-constr-count removes the limit.
* Refactoring in specLoop and friends; new algebraic data types
OneSpec and SpecInfo instead of the tuples that were there before
* Be less keen to specialise on variables that are simply in scope.
Example
f p q = letrec g a y = ...g.... in g q p
We probably do not want to specialise 'g' for calls with exactly
the arguments 'q' and 'p', since we know nothing about them.