Check whether MCInst operand isImm before calling getImm.

When processing possible aliases, TableGen assumes that if an operand *can* be
an immediate, then it always *will* be. This is incorrect for the AArch64
backend. This patch inserts a check in the generated code to make sure isImm is
true first.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171972 91177308-0d34-0410-b5e6-96231b3b80d8