[ARM] Assembler: Add more negative tests for ARM LDRD.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191664 91177308-0d34-0410-b5e6-96231b3b80d8
Fix a bug in InstCombine where it attempted to cast a Value* to an Instruction*
when it was actually a Constant*.

There are quite a few other casts to Instruction that might have the same problem,
but this is the only one I have a test case for.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191668 91177308-0d34-0410-b5e6-96231b3b80d8