ree: Fix -fcompare-debug issues in combine_reaching_defs [PR108573]

ree: Fix -fcompare-debug issues in combine_reaching_defs [PR108573]

The PR78437 r7-4871 changes made combine_reaching_defs punt on
WORD_REGISTER_OPERATIONS targets if a setter of smaller than word
register has wider uses.  This unfortunately breaks -fcompare-debug,
because if such a use appears only in DEBUG_INSN(s), while all other
uses aren't wider than the setter, we can REE optimize it without -g
and not with -g.

Such decisions shouldn't be based on debug instructions.  We could try
to reset them or adjust in some other way after we decide to perform the
change, but at least on the testcase which used to fail on riscv64-linux
the
(debug_insn 8 7 9 2 (var_location:HI s (minus:HI (subreg:HI (and:DI (reg:DI 10 a0 [160])
                (const_int 1 [0x1])) 0)
        (subreg:HI (ashiftrt:DI (reg/v:DI 9 s1 [orig:151 l ] [151])
                (debug_expr:SI D#1)) 0))) "pr108573.c":12:5 -1
     (nil))
clearly doesn't care about the upper bits and I have hard time imaging how
could one end up with DEBUG_INSN which actually cares about those upper
bits.

So, the following patch just ignores uses on DEBUG_INSNs in this case,
if we run into something where we'd need to do something further later on,
let's deal with it when we have a testcase for it.

2023-02-01  Jakub Jelinek  <jakub@redhat.com>

	PR debug/108573
	* ree.cc (combine_reaching_defs): Don't return false for paradoxical
	subregs in DEBUG_INSNs.

	* gcc.dg/pr108573.c: New test.

(cherry picked from commit e4473d7cf871c8ddf8f22d105c5af6375ebe37bf)