[PowerPC] folding rlwinm + rlwinm to rlwinm

  For example:
    x3 = rlwinm x3, 27, 5, 31
    x3 = rlwinm x3, 19, 0, 12
  can be combined to
    x3 = rlwinm x3, 14, 0, 12

Reviewed by: steven.zhang, lkail

Differential Revision: https://reviews.llvm.org/D70374