[RFC] use templated isl types Scop::makeContext

Templated isl types require the user to specify the domain and range
universes of isl objects, allowing the compiler to check whether
it makes sense to combine pairs of objects.
This RFC only converts isPromotableToRegistersBelow and some related
functions to illustrate the effect.
The isPromotableToRegistersBelow was already applying operations
correctly, so the code itself did not require any changes.
However, one variable was reused to store different types
of intermediate result and this one had to be split up into
several variables because they now have different types.

Author: Sven Verdoolaege

tc/core/polyhedral/scop.h

@@ -131,7 +131,7 @@ struct Scop {
   // Returns a set that specializes the named scop's subset of
   // parameter space to the integer values passed to the function.
   template <typename T>
-  isl::set makeContext(
+  isl::Set<> makeContext(
       const std::unordered_map<std::string, T>& sizes =
           std::unordered_map<std::string, T>()) const {
     auto s = isl::Space<>(domain().get_space());