[XCOFF][AIX] Peephole optimization for toc-data.

Followup to D101178 - peephole optimization that converts a
load address instruction and a consuming load/store into just the
load/store when its safe to do so.

eg: converts the 2 instruction code sequence
  la 4, i[TD](2)
  stw 3, 0(4)
to
  stw 3, i[TD](2)

Differential Revision: https://reviews.llvm.org/D101470