[DAGCombine] Match more patterns for half word bswap

Summary: It ensures that the bswap is generated even when a part of the subtree already matches a bswap transform.

Reviewers: craig.topper, efriedma, RKSimon, lebedev.ri

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68250

llvm-svn: 374340