[DAGCombine] Permit combining of shuffles of equivalent splat BUILD_VECTORs

[DAGCombine] Permit combining of shuffles of equivalent splat BUILD_VECTORs

combineShuffleOfScalars is very conservative about shuffled BUILD_VECTORs that can be combined together.

This patch adds one additional case - if both BUILD_VECTORs represent splats of the same scalar value but with different UNDEF elements, then we should create a single splat BUILD_VECTOR, sharing only the UNDEF elements defined by the shuffle mask.

Differential Revision: https://reviews.llvm.org/D38696

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316331 91177308-0d34-0410-b5e6-96231b3b80d8