[APInt] Remove calls to clearUnusedBits from XorSlowCase and operator^=

Summary:
There's a comment in XorSlowCase that says "0^0==1" which isn't true. 0 xored with 0 is still 0. So I don't think we need to clear any unused bits here.

Now there is no difference between XorSlowCase and AndSlowCase/OrSlowCase other than the operation being performed

Reviewers: majnemer, MatzeB, chandlerc, bkramer

Reviewed By: MatzeB

Subscribers: chfast, llvm-commits

Differential Revision: https://reviews.llvm.org/D28986

llvm-svn: 292873