i386, df: Fix up gcc.c-torture/compile/20051216-1.c -O1 -march=cascadelake

i386, df: Fix up gcc.c-torture/compile/20051216-1.c -O1 -march=cascadelake

>     rtl-optimization/98863 - tame i386 specific RPAD pass
>
> caused
>
> FAIL: gcc.c-torture/compile/20051216-1.c   -O1  (internal compiler error)
> FAIL: gcc.c-torture/compile/20051216-1.c   -O1  (test for excess errors)

The problem is that we don't revert the df flags back.
This patch fixes it by clearing DF_DEFER_INSN_RESCAN after
calling df_process_deferred_rescans, so that it doesn't leak into following
unprepared passes that expect non-deferred rescans.

2021-01-30  Jakub Jelinek  <jakub@redhat.com>

	* config/i386/i386-features.c (remove_partial_avx_dependency): Clear
	DF_DEFER_INSN_RESCAN after calling df_process_deferred_rescans.

	* gcc.target/i386/20051216-1.c: New test.