[X86] getShuffleScalarElt - add CONCAT_VECTORS/INSERT_VECTOR_ELT support.

This helped fix some i686 vXi64 broadcast folds that were becoming v2Xi32 broadcasts because we didn't match the broadcast until after SimplifyDemandedBits worked out we only used the bottom 32-bits in PMUL(U)DQ and type legalization had split the original i64 load.

A couple of regressions occurred which required some fixups - adding concat_vectors(broadcast_load,broadcast_load) splat support and recognising (unnecessary) unary shuffles of already broadcasted vectors.

This came about as part of the work investigating vector load combining from shuffles for PR42550.