DCE fix

[Jan Hubicka <hubicka at ucw dot cz>]

Hi,
the problem exposed in testcase is caused by PHI in dead BB defining
virtual operand used by dead stmt.  This dead stmt define vop used by
live stmt.

We handle the first case by replacing uses by SSA_NAME's var and sending
symbol for renaming, but we do that only for live statements, so we skip
the dead stmt.  When removing the dead stmt we compensate by replacing
operands of live stmt by vuse patching back the dead stmt.

THe patch simply sends for updaiting all uses, dead or alive.  It woudl
be also possible to do more work and do updating only in scenario
described above.

Bootstrapped/regtested x86_64-linux, OK?

struct S0 {
  signed char f0;
  signed char f4;
};

struct S0 g_19 = {
  7L, 0L
};

struct S0 g_39 = {
  0L, 0x49L
};

struct S0 g_90 = {
  1L, 0
};

signed char g_125;
signed char g_198;
signed char g_218;

signed char func_17 (int p_18, struct S0 p_20, signed char p_22, signed char
p_24)
{
  return p_20.f0;
}

signed char func_61 (int p_63, signed char p_64, signed char p_65)
{
  return 0;
}

struct S0 func_176 (int p_178, int p_179)
{
  return g_90;
}

struct S0 func_160 (int p_161, int p_163)
{
  struct S0 l_240 = {
    9L, 0L
  };
  if (func_17 (1, func_176 (g_218, 1), 1, 1))
    {
      struct S0 l_253 = {
        0xE0, 0xB4L
      };
      return l_253;
    }
  return l_240;
}

struct S0 func_150 (int p_151, signed char p_153, signed char p_154, signed
char p_155)
{
  signed char l_156 = 0;
  signed char l_266 = 0;
  if (func_17
      (func_61
       (g_125, l_156,
        func_61 (1, func_17 (1, func_160 (l_266, g_90.f4), p_153, 1), 1)),
       func_176 (g_39.f0, 1), 1, 1))
    {
      g_198 = 1;
    }
  return g_19;
}
	PR tree-optimization/40759
	* tree-ssa-dce.c (mark_virtual_phi_result_for_renaming): Mark all uses
	for renaming.
Index: tree-ssa-dce.c
===================================================================
*** tree-ssa-dce.c	(revision 150009)
--- tree-ssa-dce.c	(working copy)
*************** mark_virtual_phi_result_for_renaming (gi
*** 828,836 ****
      }
    FOR_EACH_IMM_USE_STMT (stmt, iter, gimple_phi_result (phi))
      {
-       if (gimple_code (stmt) != GIMPLE_PHI
- 	  && !gimple_plf (stmt, STMT_NECESSARY))
-         continue;
        FOR_EACH_IMM_USE_ON_STMT (use_p, iter)
          SET_USE (use_p, SSA_NAME_VAR (gimple_phi_result (phi)));
        update_stmt (stmt);
--- 828,833 ----