[DAGCombine]  A shuffle of a splat is always the splat itself

Summary:
Add a simplification:
shuffle (splat-shuffle), undef, M --> splat-shuffle

Fixes pr32449

Patch by Sanjay Patel

Reviewers: eli.friedman, RKSimon, spatel

Reviewed By: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31426

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@299047 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed