Fix a major copy'n'paste error in LLVM CodeGen

Authored by monoidal on Jul 22 2018, 10:33 AM.

Description

Fix a major copy'n'paste error in LLVM CodeGen

Summary:
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)