testsuite: Explicitly enable cselim pass for cselim-2.c

The cselim pass is enabled only for targets that have conditional move
instructions.  Since pru-unknown-elf doesn't have such instructions, the
pass is not executed, and the test fails with:

  gcc.dg/tree-ssa/cselim-2.c: dump file does not exist
  UNRESOLVED: gcc.dg/tree-ssa/cselim-2.c scan-tree-dump cselim "if-then-else store replacement: 3"

Fix by explicitly enabling the cselim pass for this test.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/cselim-2.c: Pass -ftree-cselim option.

Signed-off-by: Dimitar Dimitrov <dimitar@dinux.eu>

gcc/testsuite/gcc.dg/tree-ssa/cselim-2.c

@@ -1,5 +1,5 @@
 /* { dg-do compile } */
-/* { dg-options "-O2 -fdump-tree-cselim-stats -fno-ssa-phiopt" } */
+/* { dg-options "-O2 -ftree-cselim -fdump-tree-cselim-stats -fno-ssa-phiopt" } */
 
 struct Loc {
     int x[3];