[TargetLowering] try harder to determine undef elements of vector binops

This might be the start of tracking all vector element constants generally if we take it to its 
logical conclusion, but let's stop here and make sure this is correct/beneficial so far.

The affected tests require a convoluted path before they get simplified currently because we 
don't call SimplifyDemandedVectorElts() from binops directly and don't modify the binop operands 
directly in SimplifyDemandedVectorElts().

That's why the tests all have a trailing shuffle to induce a chain reaction of transforms. So 
something like this is happening:

1. Improve the knowledge of undefs in the binop via a SimplifyDemandedVectorElts() call that 
   originates from a shuffle.
2. Transfer that undef knowledge back to the shuffle mask user as more undef lanes.
3. Combine the modified shuffle by calling SimplifyDemandedVectorElts() again.
4. Translate the improved shuffle mask as undemanded lanes of build vector constants causing 
   those to become full undef constants.
5. Simplify the binop now that it has a full undef operand.

As we can see from the unchanged 'and' and 'or' tests, tracking undefs alone isn't a full solution. 
We would need to track zero and all-ones constants to improve those opcodes. We'd probably need to 
track NaN for FP ops too (assuming we don't have fast-math-flags set).

Differential Revision: https://reviews.llvm.org/D57066

llvm-svn: 352880