[DAGCombine] Fold Splat(bitcast(buildvector(x,..))) to splat(x)

This adds a fold which teaches the backend to fold
splat(bitcast(buildvector(x,..))) or
splat(bitcast(scalar_to_vector(x))) to a single splat.

This only handles lane 0 splats, which are only valid under LE, and
needs to be a little careful with the types it creates for the new
buildvector.

Differential Revision: https://reviews.llvm.org/D139611