i386: Fix split_double_mode with paradoxical subreg [PR100626]

split_double_mode calls simplify_gen_subreg, which fails for the
high half of the paradoxical subreg.  Return temporary register
instead of NULL RTX in this case.

2021-05-18  Uroš Bizjak  <ubizjak@gmail.com>

gcc/
	PR target/100626
	* config/i386/i386-expand.c (split_double_mode): Return
	temporary register when simplify_gen_subreg fails with
	the high half od the paradoxical subreg.