[DAGCombiner] isAllOnesConstantOrAllOnesSplatConstant(): look through bitcasts

Summary:
As pointed out in D46528, we errneously transform cases like `xor X, -1`,
even though we use said function.
It's because the `-1` is actually a bitcast there.
So i think we can just look through it in the function.

Differential Revision: https://reviews.llvm.org/D47156

llvm-svn: 332905