[x86] Teach the new vector shuffle lowering to use VPERMILPD for
single-input shuffles with doubles. This allows them to fold memory
operands into the shuffle, etc. This is just the analog to the v4f32
case in my prior commit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218193 91177308-0d34-0410-b5e6-96231b3b80d8
mop up: "Don’t duplicate function or class name at the beginning of the comment."


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218194 91177308-0d34-0410-b5e6-96231b3b80d8