gcc.dg/tree-ssa/reassoc-20.c: Adjust for mmix.

The expression of interest looks like "e_5 = a::0_1 + b::1_2;" for
mmix-knuth-mmixware, while other targets have a "." instead of the
"::".  ISTM the most useful change here is not to disable the test,
but to add an optional character in the matched expression to match
the "extra" ":".  Also checked cris-elf to assert that the expression
still passes for an "ordinary" ELF target.

gcc/testsuite:
	* gcc.dg/tree-ssa/reassoc-20.c: Adjust for mmix.

gcc/testsuite/gcc.dg/tree-ssa/reassoc-20.c

@@ -15,5 +15,5 @@ int main(void)
   printf ("%d %d\n", e, f);
 }
 
-/* { dg-final { scan-tree-dump-times "\[ab\].._. \\\+ \[ab\].._." 1 "optimized" } } */
+/* { dg-final { scan-tree-dump-times "\[ab\].\?.._. \\\+ \[ab\].\?.._." 1 "optimized" } } */
 /* { dg-final { scan-tree-dump-times " \\\+ " 2 "optimized" } } */