loop-invariant: Fix -fcompare-debug failure [PR103837]

In the following testcase we have a -fcompare-debug failure, because
can_move_invariant_reg doesn't ignore DEBUG_INSNs in its decisions.
In the testcase we have due to uninitialized variable:
  loop_header
    debug_insn using pseudo84
    pseudo84 = invariant
    insn using pseudo84
  end loop
and with -g decide not to move the pseudo84 = invariant before the
loop header; in this case not resetting the debug insns might be fine.
But, we could have also:
  pseudo84 = whatever
  loop_header
    debug_insn using pseudo84
    pseudo84 = invariant
    insn using pseudo84
  end loop
and in that case not resetting the debug insns would result in wrong-debug.
And, we don't really have generally a good substitution on what pseudo84
contains, it could inherit various values from different paths.
So, the following patch ignores DEBUG_INSNs in the decisions, and if there
are any that previously prevented the optimization, resets them before
return true.

2021-12-28  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/103837
	* loop-invariant.c (can_move_invariant_reg): Ignore DEBUG_INSNs in
	the decisions whether to return false or continue and right before
	returning true reset those debug insns that previously caused
	returning false.

	* gcc.dg/pr103837.c: New test.