Add TRI::getCallPreservedMask() hook.

Add TRI::getCallPreservedMask() hook.

The hook returns a bit-mask of call-preserved registers that will
eventually replace the current list of implicit defs on call
instructions.  This will make it possible to support multiple calling
conventions without duplicating call instruction descriptors.

The call-preserved mask is slightly different from the list returned by
the getCalleeSavedRegs() hook, it includes all aliases that are
preserved by calls.

The hook takes a CallingConv::ID argument instead of a MachineFunction
pointer, so it can provide information about calls to extern functions,
and even indirect function calls.

TRI::getCalleeSavedRegs() returns information about the function
currently being compiled. TRI::getCallPreservedMask() returns
information about the functions it is calling.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@148165 91177308-0d34-0410-b5e6-96231b3b80d8