[InstCombine] Fix assert condition in foldSelectShuffleOfSelectShuffle
#5648
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Bug introduced in e239198. 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
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Bug introduced in e239198.
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