[InstSimplify] add nsw/nuw (xor X, signbit), signbit --> X

The change to InstCombine in:
https://reviews.llvm.org/D29729
...exposes this missing fold in InstSimplify, so adding this
first to avoid a regression.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@295573 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed