[x86] Remove some essentially lying comments from the v4f64 path of the
new vector shuffle lowering.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218204 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Switch the blend implementation to use a MVT switch rather than
awkward conditions. The readability improvement of this will be even
more important as I generalize it to handle more types.

No functionality changed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218205 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Re-generate a bunch of the v4f64 test cases with my new script.

This expands the integer cases to cover the fact that AVX2 moves their
lane-crossing shuffles into the integer domain. It also adds proper
support for AVX2 run lines and the "ALL" group when it doesn't matter.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218206 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Add some more comprehensive tests for v4f64 blending.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218207 91177308-0d34-0410-b5e6-96231b3b80d8