[DAGCombiner] Replace two unchecked dyn_casts with casts.
[DAGCombiner] Replace two unchecked dyn_casts with casts.

The results of the dyn_casts were immediately dereferenced on the next line
so they had better not be null.

I don't think there's any way for these dyn_casts to fail, so use a cast
of adding null check.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@362315 91177308-0d34-0410-b5e6-96231b3b80d8

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