[PowerPC] Refactor FinishCall. [NFC]

Refactor FinishCall to be more easily understandable as a precursor to
implementing indirect calls for AIX. The refactor tries to group similar
code together at the cost of some code duplication. The high level
overview of the refactor:

- Adds a number of helper functions for things like:
  * Determining if a call is indirect.
  * What the Opcode for a call is.
  * Transforming the callee for a direct function call.
  * Extracting the Chain operand from a CallSeqStart node.
  * Building the operands of the call.

- Adds helpers for building the indirect call DAG nodes
  (excluding the call instruction itself which is created in
  `FinishCall`).

- Removes PrepareCall, which has been subsumed by the
  helpers.

- Rename 'InFlag' to 'Glue'.

- FinishCall has been refactored to:
  1) Set TOC pointer usage on the DAG for the TOC based
     subtargets.
  2) Calculate if a call is indirect.
  3) Determine the Opcode to use for the call
     instruction.
  4) Transform the Callee for direct calls, or build
     the DAG nodes for indirect calls.
  5) Buildup the call operands.
  6) Emit the call instruction.
  7) If needed, emit the callSeqEnd Node and
     finish lowering by calling `LowerCallResult`

Differential Revision: https://reviews.llvm.org/D70126