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
