1. Remove redundant calls to clearUsedBits().

1. Remove redundant calls to clearUsedBits().
2. Fix countTrailingZeros to use a faster algorithm.
3. Simplify sext() slightly by using isNegative().
4. Implement ashr using word-at-a-time logic instead of bit-at-a-time
5. Rename locals named isNegative so they don't clash with method name.
6. Fix fromString to compute negated value correctly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@34629 91177308-0d34-0410-b5e6-96231b3b80d8