[PowerPC] combine rlwinm+rlwinm to rlwinm

combine
x3 = rlwinm x3, 27, 5, 31
x3 = rlwinm x3, 19, 0, 12

to
x3 = rlwinm x3, 14, 0, 12

Reviewed by: steven.zhang

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