combineShuffleOfSplatVal previously only combined shuffles of splat
values that were themselves shuffles. This patch generalizes the
combine to also combine away shuffles of arbitrary splat values
recognized by SelectionDAG::isSplatValue, as long as doing so does not
create any new undefined lanes.
On the WebAssembly side, this patch also introduces a new custom
combine to remove undefined lanes from splatting
build_vectors. Without this extra combine, the new generic shuffle
combine would be inhibited on interesting cases such as the
shl_abs_add function in simd-shift-complex-splats.ll because it would
expose the undefined lanes.
Depends on D83605.