[patch] Enable expand_simple_operations look through single-argument phi nodes

[Zdenek Dvorak <rakdver at atrey dot karlin dot mff dot cuni dot cz>]

Hello,

this is another small problem I noticed in the testcase for PR26304:
phi nodes with a single argument sometimes appear in the code.  These
are usually eliminated in the next copy propagation pass, but in the
meantime they may manage to confuse # of iterations analysis -- in the
testcase for this pr, the loop

for (i = 0; i < 6; i++)

becomes

i' = phi(0)
for (i = i'; i < 6; i++)

and # of iterations analysis is no longer able to determine that the
number of iterations of this loop is 6.

This patch fixes the problem by making expand_simple_operations handle
such phi nodes.  Bootstrapped & regtested on i686 and ppc.

Zdenek

	* tree-ssa-loop-niter.c (expand_simple_operations): Go through
	phi nodes with just a single argument.

Index: tree-ssa-loop-niter.c
===================================================================
*** tree-ssa-loop-niter.c	(revision 113549)
--- tree-ssa-loop-niter.c	(working copy)
*************** expand_simple_operations (tree expr)
*** 744,753 ****
      return expr;
  
    stmt = SSA_NAME_DEF_STMT (expr);
!   if (TREE_CODE (stmt) != MODIFY_EXPR)
      return expr;
  
-   e = TREE_OPERAND (stmt, 1);
    if (/* Casts are simple.  */
        TREE_CODE (e) != NOP_EXPR
        && TREE_CODE (e) != CONVERT_EXPR
--- 744,770 ----
      return expr;
  
    stmt = SSA_NAME_DEF_STMT (expr);
!   if (TREE_CODE (stmt) == MODIFY_EXPR)
!     e = TREE_OPERAND (stmt, 1);
!   else if (TREE_CODE (stmt) == PHI_NODE
! 	   && PHI_NUM_ARGS (stmt) == 1)
!     {
!       e = PHI_ARG_DEF (stmt, 0);
! 
!       /* Do not pass through loop exit phi nodes.  */
!       if (current_loops && TREE_CODE (e) == SSA_NAME)
! 	{
! 	  basic_block def_bb = bb_for_stmt (SSA_NAME_DEF_STMT (e));
! 	  basic_block phi_bb = bb_for_stmt (stmt);
! 
! 	  if (def_bb
! 	      && flow_loop_nested_p (phi_bb->loop_father, def_bb->loop_father))
! 	    return expr;
! 	}
!     }
!   else
      return expr;
  
    if (/* Casts are simple.  */
        TREE_CODE (e) != NOP_EXPR
        && TREE_CODE (e) != CONVERT_EXPR