OpChain has subclasses, so add a virtual destructor.
OpChain has subclasses, so add a virtual destructor.

Summary:
OpChain has subclasses, so add a virtual destructor.

This fixes an issue when deleting subclasses of OpChain (see MatchSMLAD() specifically) in r337701.

Reviewers: javed.absar

Subscribers: llvm-commits, SjoerdMeijer, samparker

Differential Revision: https://reviews.llvm.org/D49681

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@337713 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm