[PATCH] Fix PR 18725, ICE with trivial invalid inline-asm

[Andrew Pinski <pinskia at physics dot uc dot edu>]

The problem is that we do not say that error_mark_node can be shared
even though it is the only one that should exist.
This patch fixes the problem.  I had meant to check this testcase
when I was fixing the other one but I must have missed this one.

OK? Bootstrapped and tested on ppc-darwin with no regressions.

Thanks,
Andrew Pinski

ChangeLog:
	* tree-cfg.c (tree_node_can_be_shared): Error_mark_node can be
	shared always.

Testcase:
/* { dg-do compile } */
/* { dg-options "" } */

void
f (void)
{
  int x = 1;
  asm ("" : "" (x), "" (x)); /* { dg-error "output operand constraint 
lacks" } */
}

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