Fix an assertion introduced by my last change to the toString method. We

Fix an assertion introduced by my last change to the toString method. We
can't use getZExtValue() to extract the low order bits for each digit.
Instead, we need to access the low order word directly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@37242 91177308-0d34-0410-b5e6-96231b3b80d8