[DAG] More aggressively (extract_vector_elt (build_vector x, y), c) iff element is...

[DAG] More aggressively (extract_vector_elt (build_vector x, y), c) iff element is zero constant

We currently don't extract vector elements from multi-use build vectors unless TLI.aggressivelyPreferBuildVectorSources accepts them, which seems a little extreme for constant build vectors (especially as under some cases ComputeKnownBits will indirectly extract the data for us).

This is causing a few regressions in some upcoming SimplifyDemandedBits work I'm looking at, all of which just need to know that the element is zero, so I've tweaked the fold to accept zero elements as well, which will typically fold very easily.

Differential Revision: https://reviews.llvm.org/D155582