[PATCH] D121002: [DAG] Don't convert undef to 0 when creating buildvector

[Dave Green via Phabricator via llvm-commits]

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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.


https://reviews.llvm.org/D121002

Files:
  llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  llvm/test/CodeGen/AArch64/arm64-vshuffle.ll
  llvm/test/CodeGen/PowerPC/vec-itofp.ll
  llvm/test/CodeGen/PowerPC/vec_conv_i16_to_fp64_elts.ll
  llvm/test/CodeGen/PowerPC/vec_conv_i8_to_fp32_elts.ll
  llvm/test/CodeGen/PowerPC/vec_conv_i8_to_fp64_elts.ll
  llvm/test/CodeGen/Thumb2/mve-vst3.ll

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