[CGP] r205941 changed the logic, so that a cast happens *before* 'Result' is

[CGP] r205941 changed the logic, so that a cast happens *before* 'Result' is
compared to 'AddrMode.BaseReg'. In the case that 'AddrMode.BaseReg' is
nullptr, 'Result' will also be nullptr, so the cast causes an assertion. We
should use dyn_cast_or_null here to check 'Result' is not null and it is an
instruction.

Bug found by Mats Petersson, and I reduced his IR to get a test case.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208705 91177308-0d34-0410-b5e6-96231b3b80d8