[X86] In lowerVSELECTtoVectorShuffle, don't map undef select condition to undef in shuffle mask.

Undef in select condition means we should pick the element from one side or the other. An undef in a shuffle mask means pick any element from either source or worse.

I suspect by the time we get here most of the undefs in a constant vector have been removed by other things, but doing this for safety.

llvm-svn: 325394