RISC-V Regression: Fix FAIL of predcom-2.c

Like GCN, add -fno-tree-vectorize.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/predcom-2.c: Add riscv.

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

 /* { dg-do run } */
 /* { dg-options "-O2 -funroll-loops --param max-unroll-times=8 -fpredictive-commoning -fdump-tree-pcom-details-blocks -fno-tree-pre" } */
-/* { dg-additional-options "-fno-tree-vectorize" { target amdgcn-*-* } } */
+/* { dg-additional-options "-fno-tree-vectorize" { target amdgcn-*-* riscv*-*-* } } */
 
 void abort (void);