[PPC][AIX] Fix toc-data peephole bug and some related cleanup.

Set the ReplaceFlags variable to false, since there is code meant only
for the ADDItocHi/ADDItocL nodes. This has the side effect of disabling
the peephole when the load/store instruction has a non-zero offset.
This patch also fixes retrieving the `ImmOpnd` node from the AIX small
code model pseduos and does the same for the register operand node.
This allows cleaning up the later calls to replaceOperands.
Finally move calculating the MaxOffset into the code guarded by
ReplaceFlags as it is only used there and the comment is specific to the ELF
ABI.

Fixes https://github.com/llvm/llvm-project/issues/63927

Differential Revision: https://reviews.llvm.org/D155957

(cherry picked from commit b37c7ed0)