RISC-V: NO_WARNING preferred else value for RVV

PR target/115840.

In riscv_preferred_else_value, we create an uninitialized tmp var
for else value, instead of the 0 (as default_preferred_else_value)
or the pre-exists VAR (as aarch64 does), so that we can use agnostic
policy.

The problem is that `warn_uninit` will emit a warning:
  '({anonymous})' may be used uninitialized

Let's mark this tmp var as NO_WARNING.

This problem is found when I try to build glibc with V extension.

gcc

	PR target/115840
	* config/riscv/riscv.cc(riscv_preferred_else_value): Mark
	tmp_var as NO_WARNING.

gcc/testsuite
	* gcc.dg/vect/pr115840.c: New testcase.

gcc/config/riscv/riscv.cc

@@ -11432,7 +11432,11 @@ riscv_preferred_else_value (unsigned ifn, tree vectype, unsigned int nops,
 			    tree *ops)
 {
   if (riscv_v_ext_mode_p (TYPE_MODE (vectype)))
-    return get_or_create_ssa_default_def (cfun, create_tmp_var (vectype));
+    {
+      tree tmp_var = create_tmp_var (vectype);
+      TREE_NO_WARNING (tmp_var) = 1;
+      return get_or_create_ssa_default_def (cfun, tmp_var);
+    }
 
   return default_preferred_else_value (ifn, vectype, nops, ops);
 }

gcc/testsuite/gcc.dg/vect/pr115840.c

@@ -0,0 +1,11 @@
+/* { dg-do compile } */
+/* { dg-additional-options "-Wall -Werror" } */
+
+double loads[16];
+
+void
+foo (double loadavg[], int count)
+{
+  for (int i = 0; i < count; i++)
+    loadavg[i] = loads[i] / 1.5;
+}