[APInt] Make behavior of ashr by BitWidth consistent between single and multi word.

Previously single word would always return 0 regardless of the original sign. Multi word would return all 0s or all 1s based on the original sign. Now single word takes into account the sign as well.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301159 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed