8338019: Fix simple -Wzero-as-null-pointer-constant warnings in riscv code

Reviewed-by: fyang, mli, luhenry

Author: zifeihan

src/hotspot/cpu/riscv/c1_LIRAssembler_riscv.cpp

@@ -542,7 +542,7 @@ void LIR_Assembler::const2mem(LIR_Opr src, LIR_Opr dest, BasicType type, CodeEmi
       insn = &MacroAssembler::sw; break;
     case T_OBJECT:    // fall through
     case T_ARRAY:
-      assert(c->as_jobject() == 0, "should be");
+      assert(c->as_jobject() == nullptr, "should be");
       if (UseCompressedOops && !wide) {
         insn = &MacroAssembler::sw;
       } else {

src/hotspot/cpu/riscv/c1_Runtime1_riscv.cpp

@@ -1066,4 +1066,4 @@ OopMapSet* Runtime1::generate_code_for(StubID id, StubAssembler* sasm) {
 
 #undef __
 
-const char *Runtime1::pd_name_for_address(address entry) { Unimplemented(); return 0; }
+const char *Runtime1::pd_name_for_address(address entry) { Unimplemented(); }

src/hotspot/cpu/riscv/frame_riscv.cpp

@@ -269,7 +269,7 @@ void frame::patch_pc(Thread* thread, address pc) {
 
   // Either the return address is the original one or we are going to
   // patch in the same address that's already there.
-  assert(_pc == pc_old || pc == pc_old || pc_old == 0, "must be");
+  assert(_pc == pc_old || pc == pc_old || pc_old == nullptr, "must be");
   DEBUG_ONLY(address old_pc = _pc;)
   *pc_addr = pc;
   _pc = pc; // must be set before call to get_deopt_original_pc

src/hotspot/cpu/riscv/nativeInst_riscv.cpp

@@ -685,7 +685,7 @@ address NativeJump::jump_destination() const {
   // load
 
   // return -1 if jump to self or to 0
-  if ((dest == (address) this) || dest == 0) {
+  if ((dest == (address) this) || dest == nullptr) {
     dest = (address) -1;
   }
 
@@ -714,7 +714,7 @@ address NativeGeneralJump::jump_destination() const {
   // a general jump
 
   // return -1 if jump to self or to 0
-  if ((dest == (address) this) || dest == 0) {
+  if ((dest == (address) this) || dest == nullptr) {
     dest = (address) -1;
   }