[X86] Add a few more fptosi test cases to demonstrate...

[X86] Add a few more fptosi test cases to demonstrate -x86-experimental-vector-widening legalization not combining vpacksswb+vpmovdw.

We are able to combine vpackuswb+vpmovdw, but we didn't have packsswb+vpmovdw at the time that combine was added.

llvm-svn: 348909