[X86] Change the pre-sse4.1 code in the v16i8 MULHU lowering to be what we get after DAG combine cleans it up.
[X86] Change the pre-sse4.1 code in the v16i8 MULHU lowering to be what we get after DAG combine cleans it up.

Previously we emitted a punpcklbw/punpckhbw to move the byte elements into the upper half of 16 bit elements then shifted right by 8 to zero the upper bits. After DAG combine we end up with punpcklbw/punpckhbw into the lower bits with zeros in the uppers bits and no shifts. So just emit that directly.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@347966 91177308-0d34-0410-b5e6-96231b3b80d8

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