For PR1117:

For PR1117:
Make the assembler generate a nice error message if a bad cast instruction
is attempted instead of asserting out. This is made possible by the
recently exposed method CastInst::castIsValid() which checks the validity
of any cast instruction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@33283 91177308-0d34-0410-b5e6-96231b3b80d8