DI: Strengthen some dyn_casts to DIDerivedType, NFC
DI: Strengthen some dyn_casts to DIDerivedType, NFC

The surrounding code proves in both cases that these must be
`DIDerivedType` if they're `DIDerivedTypeBase`, so strengthen the
`dyn_cast`s to the more specific type.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@243143 91177308-0d34-0410-b5e6-96231b3b80d8

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