[x86] As a follow-up to r217819, don't check for VSELECT legality now
that we don't use VSELECT and directly emit an addsub synthetic node.
Also remove a stale comment referencing VSELECT.

The test case is updated to use 'core2' which only has SSE3, not SSE4.1,
and it still passes. Previously it would not because we lacked
sufficient blend support to legalize the VSELECT.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217849 91177308-0d34-0410-b5e6-96231b3b80d8
[FastISel][AArch64] Add vector support to argument lowering.

Lower the first 8 vector arguments too.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217850 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Remove the last vestiges of the BLENDI-based ADDSUB pattern
matching. This design just fundamentally didn't work because ADDSUB is
available prior to any legal lowerings of BLENDI nodes. Instead, we have
a dedicated ADDSUB synthetic ISD node which is pattern matched trivially
into the instructions. These nodes are then recognized by both the
existing and a trivial new lowering combine in the backend. Removing
these patterns required adding 2 missing shuffle masks to the DAG
combine, without which tests would have failed. Added the masks and
a helpful assert as well to catch if anything ever goes wrong here.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217851 91177308-0d34-0410-b5e6-96231b3b80d8