[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