[PATCH] [DagCombiner] Generalized BuildVector Vector Concatenation

[Simon Pilgrim]

Hi qcolombet, chandlerc, andreadb,

The CONCAT_VECTORS combiner pass can transform the concat of two BUILD_VECTOR nodes into a single BUILD_VECTOR node.

This patch generalises this to support any number of BUILD_VECTOR nodes, and also permits UNDEF nodes to be included as well.

This was noticed as AVX vec128 -> vec256 canonicalization sometimes creates a CONCAT_VECTOR with a real vec128 lower and an vec128 UNDEF upper (see zext test modifications).

REPOSITORY
  rL LLVM

http://reviews.llvm.org/D7816

Files:
  lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  test/CodeGen/X86/vector-zext.ll

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