[x86] Fix a miscompile in the new shuffle lowering uncovered by

[x86] Fix a miscompile in the new shuffle lowering uncovered by
a bootstrap.

I managed to mis-remember how PACKUS worked on x86, and was using undef
for the high bytes instead of zero. The fix is fairly obvious.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211922 91177308-0d34-0410-b5e6-96231b3b80d8