[X86][SSE] getFauxShuffle - account for PEXTW/PEXTB implicit zero-extension

The insert(truncate/extend(extract(vec0,c0)),vec1,c1) case in rGacbc5ede99 wasn't combining the 'mineltsize' with the src vector elt size which may be smaller due to implicit extension during extraction.

Reduced from test case provided by @mstorsjo