Fix APInt long division algorithm

Fix APInt long division algorithm

Summary: This patch fixes step D4 of Knuth's division algorithm implementation. Negative sign of the step result was not always detected due to incorrect "borrow" handling.

Test Plan: Unit test that reveals the bug included.

Reviewers: chandlerc, yaron.keren

Reviewed By: yaron.keren

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9196

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235699 91177308-0d34-0410-b5e6-96231b3b80d8