[X86] Change vXi8 MULHU lowering to unpack high and low half of lanes instead of...

[X86] Change vXi8 MULHU lowering to unpack high and low half of lanes instead of extracting and concating low and high half registers.

This reduces the number of shuffle operations that need to be done. The splitting strategy requires the shuffle unit for the extraction and the extension. With the unpack strategy the unpacks accomplish a splitting and extending in one operation.

llvm-svn: 348019