Refactor to expose RTLIB calls to targets.

fp128 is almost but not quite completely illegal as a type on AArch64. As a
result it needs to have a register class (for argument passing mainly), but all
operations need to be lowered to runtime calls. Currently there's no way for
targets to do this (without duplicating code), as the relevant functions are
hidden in SelectionDAG. This patch changes that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171971 91177308-0d34-0410-b5e6-96231b3b80d8
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