Start nodes for JS functions have the following Parameter node value
outputs:

 closure, ...args_including_receiver, new_target, argc, context

This CL adds helper functions for these. There's two interesting
gotcha's:

- Each Parameter node is associated with an index, starting at -1.
Value output indices obviously start at 0, so there's an off-by-one
between the value output of the Parameter node, and the Parameter
node's associated index.
- CSA/Torque graphs use different Start node layouts, yet these are
not reflected in compiler logic. There's potential for confusion here.
The two layouts should be unified or made explicit.

Finally, tests create Start nodes with arbitrary layouts. This blocks
removal of methods marked _MaybeNonStandardLayout.

In an ideal world, the parameter index would equal the start node
output index, and the layout of all Start nodes would be equal. Future
work..

Change-Id: I908909880817979062d459b7a80ed4fede40e2ec
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2649035
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72352}