lra-constraints: Fix error-recovery for bad inline-asms [PR97971]

lra-constraints: Fix error-recovery for bad inline-asms [PR97971]

The following testcase has ice-on-invalid, it can't be reloaded, but we
shouldn't ICE the compiler because the user typed non-sense.

In current_insn_transform we have:
  if (process_alt_operands (reused_alternative_num))
    alt_p = true;

  if (check_only_p)
    return ! alt_p || best_losers != 0;

  /* If insn is commutative (it's safe to exchange a certain pair of
     operands) then we need to try each alternative twice, the second
     time matching those two operands as if we had exchanged them.  To
     do this, really exchange them in operands.

     If we have just tried the alternatives the second time, return
     operands to normal and drop through.  */

  if (reused_alternative_num < 0 && commutative >= 0)
    {
      curr_swapped = !curr_swapped;
      if (curr_swapped)
        {
          swap_operands (commutative);
          goto try_swapped;
        }
      else
        swap_operands (commutative);
    }

  if (! alt_p && ! sec_mem_p)
    {
      /* No alternative works with reloads??  */
      if (INSN_CODE (curr_insn) >= 0)
        fatal_insn ("unable to generate reloads for:", curr_insn);
      error_for_asm (curr_insn,
                     "inconsistent operand constraints in an %<asm%>");
      lra_asm_error_p = true;
...
and so handle inline asms there differently (and delete/nullify them after
this) - fatal_insn is only called for non-inline asm.
But in process_alt_operands we do:
                /* Both the earlyclobber operand and conflicting operand
                   cannot both be user defined hard registers.  */
                if (HARD_REGISTER_P (operand_reg[i])
                    && REG_USERVAR_P (operand_reg[i])
                    && operand_reg[j] != NULL_RTX
                    && HARD_REGISTER_P (operand_reg[j])
                    && REG_USERVAR_P (operand_reg[j]))
                  fatal_insn ("unable to generate reloads for "
                              "impossible constraints:", curr_insn);
and thus ICE even for inline-asms.

I think it is inappropriate to delete/nullify the insn in
process_alt_operands, as it could be done e.g. in the check_only_p mode,
so this patch just returns false in that case, which results in the
caller have alt_p false, and as inline asm isn't simple move, sec_mem_p
will be also false (and it isn't commutative either), so for check_only_p
it will suggests to the callers it isn't ok and otherwise will emit
error and delete/nullify the inline asm insn.

2021-02-03  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/97971
	* lra-constraints.c (process_alt_operands): For inline asm, don't call
	fatal_insn, but instead return false.

	* gcc.target/i386/pr97971.c: New test.