Fix an unused variable warning from Clang by sinking a dyn_cast into an
isa and a cast inside the assert. The efficiency concern isn't really
important here. The code should likely be cleaned up a bit more,
especially getting a message into the assert.

Please review Rafael.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177053 91177308-0d34-0410-b5e6-96231b3b80d8