[DAG] Refactor the shuffle combining logic in DAGCombiner. NFC.

This patch simplifies the logic that combines a pair of shuffle nodes into
a single shuffle if there is a legal mask. Also added comments to better
describe the algorithm. No functional change intended.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222522 91177308-0d34-0410-b5e6-96231b3b80d8