[X86] Fix a couple places that were using macro arguments twice when of the...

[X86] Fix a couple places that were using macro arguments twice when of the usages could just be undefined.

One of the arguments was being used when the passthru argument is unused due to the mask being all 1s. But in that case the actual value doesn't matter so we should use undef instead to avoid expanding the macro argument unnecessarily.

llvm-svn: 333865