[X86][SSE] combineX86ShufflesConstants - early out for zeroable vectors (PR45443)

Shuffle combining can insert zero byte sized elements into the shuffle mask, which combineX86ShufflesConstants will attempt to fold without taking into account whether the byte-sized type is legal (e.g. AVX512F only targets).

If we have a full-zeroable vector then we should just return a zero version of the root type, otherwise if the type isn't valid we should bail.

Fixes PR45443