[AArch64] Fix unintended fallthrough and strengthen cast
[AArch64] Fix unintended fallthrough and strengthen cast

This was added in r330630. GCC's -Wimplicit-fallthrough seems to not
fire when the previous case contains a switch itself.

This fallthrough was bening because the helper function implementing the
case used dyn_cast to re-check the type of the node in question. After
fixing the fallthrough, we can strengthen the cast.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@345864 91177308-0d34-0410-b5e6-96231b3b80d8

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