This code works around what appears to be a bug in another part of clang.

I have filed http://llvm.org/bugs/show_bug.cgi?id=15538 against clang.
This code is safer anyway because "cast" assumes you really know that
it's okay to make the cast. In this case isa should not be false and
dyn_cast should not return null as far as I understand. But everything
else is valid so I did not want to revert my previous patch for attributes
mips16/nomips16 or use an llvm_unreachable here which would make a number
of our tests fail for mips.

llvm-svn: 177329