testsuite/gcc.dg/uninit-pred-9_b.c:20: Fix XPASS for various targets

The xfail for "*-*-*" here, set in r14-4089-gd45ddc2c04e471
"tree-optimization/111294 - backwards threader PHI costing"
was somewhat too general and made this test XPASS for a
number of targets.  The common factor for those targets is
that they either explicitly or by default define
LOGICAL_OP_NON_SHORT_CIRCUIT as 0 (see fold-const.cc).

Instead of changing *-*-* to a seemingly random set of
xfailed targets or inventing a new testsuite
effective-target predicate for logical-op-short-circuited
targets or the opposite, let's just force a setting that
removes the need for the xfail for all targets, by
overriding with --param=logical-op-non-short-circuit=0.

	* gcc.dg/uninit-pred-9_b.c: Remove xfail for line 20.  Pass
	--param=logical-op-non-short-circuit=0.  Comment why.

gcc/testsuite/gcc.dg/uninit-pred-9_b.c

@@ -1,5 +1,6 @@
 /* { dg-do compile } */
-/* { dg-options "-Wuninitialized -O2" } */
+/* The param shuts up a bogus uninitialized warning at line 21.  */
+/* { dg-options "-Wuninitialized -O2 --param=logical-op-non-short-circuit=0" } */
 
 int g;
 void bar();
@@ -17,7 +18,7 @@ int foo (int n, int l, int m, int r)
 
   if (l > 100)
     if ( (n <= 9) &&  (m < 100)  && (r < 19) )
-      blah(v); /* { dg-bogus "uninitialized" "bogus warning" { xfail *-*-* } } */
+      blah(v); /* { dg-bogus "uninitialized" "bogus warning" } */
 
   if ( (n <= 8) &&  (m < 99)  && (r < 19) )
       blah(v); /* { dg-bogus "uninitialized" "pr101674" } */