[vm/bytecode] Throw should be 'debug checked', but not ReThrow.

Since both throw and rethrow statements share the same 'Throw' bytecode opcode
(differentiated by an operand), we make the opcode not implicitly 'debug
checked' anymore.
Instead, we emit a 'DebugCheck' opcode for the throw statement only.

This fixes the following test:
service/async_single_step_exception_test

Change-Id: I4e540ea0a640fda5365d51933ce4e5125548e2f2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/114754
Commit-Queue: Régis Crelier <regis@google.com>
Reviewed-by: Alexander Markov <alexmarkov@google.com>

pkg/vm/lib/bytecode/gen_bytecode.dart

@@ -3174,6 +3174,10 @@ class BytecodeGenerator extends RecursiveVisitor<Null> {
   @override
   visitThrow(Throw node) {
     _generateNode(node.expression);
+
+    if (options.emitDebuggerStops) {
+      asm.emitDebugCheck();
+    }
     asm.emitThrow(0);
   }
 

runtime/vm/compiler/frontend/bytecode_flow_graph_builder.cc

@@ -1569,8 +1569,6 @@ void BytecodeFlowGraphBuilder::BuildThrow() {
     UNIMPLEMENTED();  // TODO(alexmarkov): interpreter
   }
 
-  BuildDebugStepCheck();
-
   if (DecodeOperandA().value() == 0) {
     // throw
     LoadStackSlots(1);

runtime/vm/constants_kbc.h

@@ -994,7 +994,6 @@ class KernelBytecode {
       case KernelBytecode::kDynamicCall:
       case KernelBytecode::kDynamicCall_Wide:
       case KernelBytecode::kReturnTOS:
-      case KernelBytecode::kThrow:
       case KernelBytecode::kEqualsNull:
       case KernelBytecode::kNegateInt:
       case KernelBytecode::kNegateDouble:

runtime/vm/interpreter.cc

@@ -1780,7 +1780,6 @@ SwitchDispatch:
 
   {
     BYTECODE(Throw, A);
-    DEBUG_CHECK;
     {
       SP[1] = 0;  // Space for result.
       Exit(thread, FP, SP + 2, pc);