[APInt] Add unittests that demonstrate how very broken APIntOps::isShiftedMask is.

Did you know that 0 is a shifted mask? But 0x0000ff00 and 0x000000ff aren't? At least we get 0xff000000 right.

I only see one usage of this function in the code base today and its in InstCombine. I think its protected against 0 being misreported as a mask. I guess we just don't have tests for the missed cases.

llvm-svn: 299187