[ValueTracking] peek through extends in haveNoCommonBitsSet

In cases with matching extends, this allows changing an 'add'
into an 'or' and narrowing the 'or' which then simplifies to
a constant.

In cases with opposite extends, we just convert to an 'or'
currently, but that could be reduced too.

https://alive2.llvm.org/ce/z/fTHzdb