[x86] simplify logic in lowerShuffleWithUndefHalf(); NFCI

[x86] simplify logic in lowerShuffleWithUndefHalf(); NFCI

This seems unnecessarily complicated because we gave names to
opposite polarity bools and have code comments that don't really
line up with the logic.

Step 1: remove UndefUpper and assert that it is the opposite of
UndefLower after the initial early exit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@352217 91177308-0d34-0410-b5e6-96231b3b80d8