Fix rare case where APInt divide algorithm applied un-needed transformation.

APInt uses Knuth's D algorithm for long division. In rare cases the
implementation applied a transformation that was not needed.

Added unit tests for long division. KnuthDiv() procedure is fully covered.
There is a case in APInt::divide() that I believe is never used (marked with
a comment) as all users of divide() handle trivial cases earlier.

Patch by Pawel Bylica!

  http://reviews.llvm.org/D8448

llvm-svn: 233312