PhaseOutput::fill_buffer detects if the real size of a node matches (<=) the size of it in scratch_emit(). The call chain for MacroAssembler::stop() is:

```
MachEpilogNode::emit
    -> reserved_stack_check()
         -> should_not_reach_here()
              -> stop(const char *msg)
```

li() on RISCV could generate 1~6 instructions, and the msg argument could be an on-stack buffer; it uses __ pc() that could also be different in scratch_emit() and emit(). They both have the potential issue here so the size generated in MacroAssembler::stop() needs to be a fixed value.

Could be reproduced in fastdebug build:

```
// Die now.
instruct ShouldNotReachHere() %{
  match(Halt);
  ins_cost(BRANCH_COST);
  format %{ "#@ShouldNotReachHere" %}
  ins_encode %{
    Assembler::CompressibleRegion cr(&_masm);
    if (is_reachable()) {
      __ halt();
+ __ unimplemented("this is an on-stack char literal"); // assertion fail at 'assert(false, "wrong size of mach node");'
    }
  %}
  ins_pipe(pipe_class_default);
%}
```

Tests passed in hotspot tier1 & jdk tier1.