Nop bytecodes are required only for break locations in debugger. Since nop bytecode doesn't change program state we can remove all of them.

There are at least two changes which this CL produce:
- we don't provide break position when we load local variable (still provide when load variable from global),
- we don't provide break position for statements without actual break positions (e.g. "a;") - these expressions should be super rare and user always can set breakpoint before or after this statement.

More details in one pager: https://docs.google.com/a/google.com/document/d/1JXlQpfMa9vRojbE272b6GMBbrfh6m_00135iAUOJEz8/edit?usp=sharing

Bug: v8:6425
Change-Id: I4aee73d497a84f7b5d89caa6dda6d3060567dfda
Reviewed-on: https://chromium-review.googlesource.com/543161Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46742}

Goal of this CL: explicit return from non-async function has position after
return expression as return position (will unblock [1]).

BytecodeArrayBuilder has SetStatementPosition and SetExpressionPosition methods.
If one of these methods is called then next generated bytecode will get passed
position. It's general treatment for most cases.
Unfortunately it doesn't work for Returns:
- debugger requires source positions exactly on kReturn bytecode in stepping
  implementation,
- BytecodeGenerator::BuildReturn and BytecodeGenerator::BuildAsyncReturn
  generates more then one bytecode and general solution will put return position
  on first generated bytecode,
- it's not easy to split BuildReturn function into two parts to allow something
  like following in BytecodeGenerator::VisitReturnStatement since generated
  bytecodes are actually controlled by execution_control().
..->BuildReturnPrologue();
..->SetReturnPosition(stmt);
..->Return();

In this CL we pass ReturnStatement through ExecutionControl and use it for
position when we emit return bytecode right here.

So this CL only will improve return position for returns inside of non-async
functions, I'll address async functions later.

[1] https://chromium-review.googlesource.com/c/543161/

Change-Id: Iede512c120b00c209990bf50c20e7d23dc0d65db
Reviewed-on: https://chromium-review.googlesource.com/560738
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46687}

This includes several changes. From most to least interesting:

- No longer implement AwaitExpressions using a do-expression.
- Reduces frame-size of async generators by not allocating temporary
  variables to hold results of Await epxressions.
- Streamline and reduce generated bytecodes for Await.
- Debugger no longer emits a debug::kCallBreakLocation breakpoint for
the JS-builtin call performed for Await, and instead only emits such
a breakpoint if the operand of Await is actually a call.
- Push fewer parameters to Await* builtins, using the receiver for the
  first parameter (possible now that the CallRuntime invocation not
  part of the AST).
- Adds a new Await AST node. No new members or anything, but it seemed
  palatable to avoid having `if (is_await())` in a number of
  VisitSuspend functions.

BUG=v8:5855, v8:5099, v8:4483
R=rmcilroy@chromium.org, kozyatinskiy@chromium.org, yangguo@chromium.org
TBR=bmeurer@chromium.org

Change-Id: I9cd3fda99cd40295c04fdf1aea01b5d83fac6caf
Reviewed-on: https://chromium-review.googlesource.com/558806
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46666}

In current implementation in expressions like await foo() we have break location right after foo call and before actual await.
And we additionally have a lot of other statement locations because of do scope.

Let's move async debugging closer to sync debugging and introduce only one break location for await - before awaited function call.

Bug: v8:6425,v8:6162
Change-Id: I7568767856022c49101e7f3b7e39a2e401d21644
Reviewed-on: https://chromium-review.googlesource.com/514046Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45625}