[x86] Fix another bug hit when bootstrapping with the new shuffle
lowering.

For maximum irony, I had already discovered this bug, diagnosed it, and
left FIXMEs about it in the test cases. =[ I just failed to go back over
those until after i had reduced a bootstrap miscompile down to a single
TU, stared at the assembly for an hour, and figured out the bug. Again.

Oh well.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211955 91177308-0d34-0410-b5e6-96231b3b80d8