[X86][SSE] combineSelect - more aggressively create zero elements in the...

[X86][SSE] combineSelect - more aggressively create zero elements in the or(pshufb(x), pshufb(y)) fold

When we fold vselect(cond, pshufb(x), pshufb(y)) -> or(pshufb(x), pshufb(y)), ensure we convert all undef elements to zero elements - this should help us expose more known zero elements for deeper chains of these cases.

Noticed while triaging Issue #54819