[PowerPC] Collect some CallLowering arguments into a struct. [NFC]

Collect the calling convention and a number of boolean arguments into a
structure to slightly reduces the number of arguments passed around between
LowerCall_<Subtarget>, FinishCall and a few of the helpers. Also
calulates if a call is indirect once using the exisitng helper and caches the
result replacing several instances where we duplicated the logic determining if
a call is indirect.