[SelectionDAG] enhance vector demanded elements to look at a vector select condition operand

This is the DAG equivalent of D51433.
If we know we're not using all vector lanes, use that knowledge to potentially simplify a vselect condition.

The reduction/horizontal tests show that we are eliminating AVX1 operations on the upper half of 256-bit 
vectors because we don't need those anyway.
I'm not sure what the pr34592 test is showing. That's run with -O0; is SimplifyDemandedVectorElts supposed 
to be running there?

Differential Revision: https://reviews.llvm.org/D51696

llvm-svn: 341762

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