[InstCombine] add helper function to foldCastedBitwiseLogic() ; NFCI

This is a straight cut and paste of the existing code and is intended to
be the first step in solving part of PR26702:
https://llvm.org/bugs/show_bug.cgi?id=26702

We should be able to reuse most of this and delete the nearly identical 
existing code in visitOr(). Then, we can enhance visitXor() to use the
same code too.

llvm-svn: 261649