[DAGCombine] Add SCALAR_TO_VECTOR undef handling to simplifyShuffleMask.

This allows us to simplify later visitVECTOR_SHUFFLE optimizations such as combineShuffleOfScalars.

Noticed whilst working on D38696

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316017 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed