[x86] Cleanup an unused variable by actually using it in the non-asserts
place where it was needed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217854 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Remove a FIXME that doesn't make any sense. Only the lanes feeding
the blend that is matched by this are "used" in any sense, and so any
build_vector or other nodes feeding these will already drop other lanes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217855 91177308-0d34-0410-b5e6-96231b3b80d8
tweak test case to help solve why failing on one build bot

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217856 91177308-0d34-0410-b5e6-96231b3b80d8