[X86] combineConcatVectorOps - don't concat(vselect,vselect) if the...

[X86] combineConcatVectorOps - don't concat(vselect,vselect) if the concatenated selection mask isn't legal

One of the crash regression tests now exposes an existing issue with SelectionDAG::simplifySelect not folding vselect with constant masks

Fixes #59003