Fix countLeadingZeros in the case that the bitwidth evenly divides the

Fix countLeadingZeros in the case that the bitwidth evenly divides the
word size. This fixes all reads of uninitialized data (buffer over read)
and makes APInt.cpp memory clean, per valgrind. The only remaining
problem is division in a few cases.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@34483 91177308-0d34-0410-b5e6-96231b3b80d8