Fix crash in vect_is_slp_reduction

[Ryan Mansfield <rmansfield@qnx.com>]

There is a crash in vect_is_slp_reduction where use_stmt doesn't get 
initialized in the FOR_EACH_IMM_USE_FAST loop.

1718	      FOR_EACH_IMM_USE_FAST (use_p, imm_iter, lhs)
(gdb) call debug_tree(lhs)
  <ssa_name 0x7ffff6e3a5a0
     type <integer_type 0x7ffff7f8c7e0 int32_t sizes-gimplified public SI
         size <integer_cst 0x7ffff7ec47e0 constant 32>
         unit size <integer_cst 0x7ffff7ec4580 constant 4>
         align 32 symtab 0 alias set -1 canonical type 0x7ffff7ecb498 
precision 32 min <integer_cst 0x7ffff7ec4780 -2147483648> max 
<integer_cst 0x7ffff7ec47a0 2147483647> context <translation_unit_decl 
0x7ffff7f98508 D.1314>
         pointer_to_this <pointer_type 0x7ffff7f9f930>>
     var <var_decl 0x7ffff6e1ce60 g_29_lsm.18>def_stmt g_29_lsm.18_137 = 
PHI <g_29_lsm.18_87(15), D.2113_96(18)>

     version 137>
(gdb) n
1742	      if (found)
(gdb)
1746	      if (gimple_code (use_stmt) == GIMPLE_PHI)
(gdb)

Program received signal SIGSEGV, Segmentation fault.
0x0000000000980f22 in gimple_code (g=0x0) at ../../gcc/gimple.h:1098
1098	  return g->gsbase.code;

Is the following sufficient?

Index: tree-vect-loop.c
===================================================================
--- tree-vect-loop.c	(revision 173945)
+++ tree-vect-loop.c	(working copy)
@@ -1743,7 +1743,7 @@
          break;

        /* This is a loop exit phi, and we haven't reached the reduction 
phi.  */
-      if (gimple_code (use_stmt) == GIMPLE_PHI)
+      if (!use_stmt || gimple_code (use_stmt) == GIMPLE_PHI)
          return false;

        if (!is_gimple_assign (use_stmt)

Regards,

Ryan Mansfield