[X86][AVX] Ensure we retain zero elements in select(pshufb,pshufb) ->...

[X86][AVX] Ensure we retain zero elements in select(pshufb,pshufb) -> or(pshufb,pshufb) fold (PR52122)

The select(pshufb,pshufb) -> or(pshufb,pshufb) fold uses getConstVector to create the refreshed pshufb masks, which treats all negative indices as undef.

PR52122 shows that if we were selecting an element that the PSHUFB has set to zero we must set it back to 0x80 when we recreate the PSHUFB mask and not just leave it as SM_SentinelZero