Fix a major copy'n'paste error in LLVM CodeGen
In D4592, AddWordC is lowered as an unsigned subtraction instead
of an unsigned addition when compiling with LLVM.
This patch rectifies that.
Reviewers: angerman, bgamari, monoidal
Reviewed By: angerman, bgamari, monoidal
Subscribers: osa1, rwbarton, thomie, carter
Differential Revision: https://phabricator.haskell.org/D4969
(cherry picked from commit f629442be93f4608e6bb53bfe2264a406230c546)