[X86][SSE] getFauxShuffleMask - peek through TRUNCATE/AEXT/ZEXT for...

[X86][SSE] getFauxShuffleMask - peek through TRUNCATE/AEXT/ZEXT for INSERT_VECTOR_ELT(EXTRACT_VECTOR_ELT())

As long we extract from a source vector with smaller elements and we zero-extend the element in the final shuffle mask then we can safely peek through truncations and any/zero-extensions to find the source extraction.