[InstCombine] Fix assert condition in `foldSelectShuffleOfSelectShuffle`

Bug introduced in e239198c.

The assert() is making an assumption that the resulting shuffle mask
will always select elements from both vectors, this is untrue in the
case of two shuffles being folded if the former shuffle has a mask with
undef elements in it. In such a case folding the shuffles might result
in a mask which only selects from one of the vectors because the other
elements (in the mask) are undef.

Differential Revision: https://reviews.llvm.org/D136256