Wait... what? "To actually compile a Haskell program, we'll also need a patched version of GHC. See Building and Using GHC with HRC for more information."
A core design principle of Haskell is that while the whole language has gotten relatively complex thanks to all its language features and extensions, almost everything can be simplified to a really small and elegant core language. This core language is a typed lambda calculus that looks a lot like a subset of Haskell except with a few changes like no type inference and different rules for strictness.
GHC then uses this pared down version of Haskell (appropriately called Core) for the rest of its optimization and compilation. This means that once the first pass is done with type inference, type checking, typeclass resolution and a lot of other high-level transformations, the rest of the compiler doesn't have to worry about them at all. This makes all of GHCs optimizations easier to implement and maintain, and it lets us add features to Haskell without needing to change the backend.
https://www.microsoft.com/en-us/research/publication/the-imp...
Or does the sugar carry no such information?
More importantly, any modern compiler is very far from the producing the most optimal possible programs. Overall performance could be improved pretty much anywhere, and the tradeoffs involved make operating on the "whole" language a far lower priority than pretty much anything else.
Parsing the language isn't a very interesting research problem, and more work than it's worth, so Intel have chosen to instead use GHC's frontend to generate Core code, and then consume Core for their compiler backend.
An excellent overview: http://www.aosabook.org/en/ghc.html