StaticAnalyzer: fix compiler warning. NFC

My compiler (clang-3.8) complains that the RCC variable is unused.
That's not really true, as it's checked by the if-declaration, but it's
also kinda true, because we don't need to declaration if we only check
it in the if statement.

In reality, all this means that the dyn_cast<> can be replaced by isa<>,
so that's what I do here.

llvm-svn: 327491