David Green 4388f4f776 [DAG] Don't convert undef to 0 when creating buildvector ...

When inserting undef into buildvectors created from shuffles of
buildvectors, we convert elements to the largest needed type. This had
the effect of converting undef into 0, which isn't needed as the
buildvector implicitly truncates and trunc(zext(undef)) == undef.

Differential Revision: https://reviews.llvm.org/D121002

2022-03-06 18:35:34 +00:00

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