[x86] Add a comment clarifying that this test should span all manners of
generic DAG combining of shuffles relevant to x86.

My plan is to fold a bunch of the other DAG combining test cases into
this one, while converting them to use the nice new FileCheck assertion
syntax.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218855 91177308-0d34-0410-b5e6-96231b3b80d8