df: Fix up handling of paradoxical subregs in debug insns [PR101170]

The recent addition of gcc_assert (regno < endregno); triggers during
glibc build on m68k.
The problem is that RA decisions shouldn't depend on expressions in
DEBUG_INSNs and those expressions can contain paradoxical subregs of certain
pseudos.  If RA then decides to allocate the pseudo to a register
with very small hard register REGNO, we can trigger the new assert,
as (int) subreg_regno_offset may be negative on big endian and the small
REGNO + the negative offset can wrap around.

The following patch in that case records the range from the REGNO 0 to
endregno, before the addition of the assert as both regno and endregno are
unsigned it wouldn't record anything at all silently.

2021-06-24  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/101170
	* df-scan.c (df_ref_record): For paradoxical big-endian SUBREGs
	where regno + subreg_regno_offset wraps around use 0 as starting
	regno.

	* gcc.dg/pr101170.c: New test.