Fix a typo: use of cast where dyn_cast was intended

This bug has the effect of converting a test of isGCRelocate(InvokeInst*) from a false return to a crash.

This may be the root cause of the crash Joerg reported against r223137, but I'm still waiting for a clean build of clang to complete to be able to confirm.  Once I've confirmed the issue, I'll submit a test case separately.