Function `expandCheckImmOperand` should always check if the input machine
instruction is passed by reference before calling method `getOperand()` on it.
Found while working on a patch that relies on `expandCheckImmOperand` to expand
a scheduling predicate.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@337294 91177308-0d34-0410-b5e6-96231b3b80d8