[X86] Change the pre-sse4.1 code in the v16i8 MULHU lowering to be what we get...

[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.

llvm-svn: 347966