[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.

llvm-svn: 211955