Also zero upper at start of function

tools/simdstring/generate.go

@@ -1048,6 +1048,10 @@ func (s *Function) OutputASM(w io.Writer) {
 	s.print_signature(w)
 	fmt.Fprintf(w, "\nTEXT ·%s(SB), NOSPLIT, $0-%d\n", s.Name, s.Size)
 
+	if s.Used256BitReg {
+		fmt.Fprintln(w, "\tVZEROUPPER // zero upper bits of AVX registers to avoid dependencies when switching between SSE and AVX code")
+	}
+
 	s.Return()
 	for _, i := range s.Instructions {
 		fmt.Fprintln(w, i)