[X86][SSE] Move VZEXT_MOVL(INSERT_SUBVECTOR(UNDEF,X,0)) handling into combineTargetShuffle.

Now that we're getting better at combining shuffles of different vector widths, this can now be performed as part of the standard target shuffle combines and isn't required for cleanup.

Exposed a minor issue in combineX86ShufflesRecursively where we failed to check if a shuffle's src ops were simple types.