[dart2wasm] Disentagle dispatch table calls from static calls in dart2wasm

Not all instance members are necessarily called via dispatch table
calls.

If we know an instance member isn't a target of any dispatch table call
we can choose the representation for that member on its own (instead of
using the selector representation - which is a upper-bounding of all
implementations of that selector).

This CL requires all call sites to obtain target signature & parameter
information via one of

  * `Translator.{signature,paramInfo}ForDirectCall`
  * `Translator.{signature,paramInfo}ForDispatchTableCall`

=> Those methods handle the case of calling a method

   a) via dispatch table call
   b) via direct call where
     b.1) target can also be called via dispatch table
     b.2) target cannot be called via dispatch table

In a follow up CL we're going to reland [0] which will shrink the set of
instance methods that are callable via dispatch table calls (e.g.
members that are only called via `super.*()` can use their own
representation selection instead of clustering it with methods of same
name that can be called via dispatch table).

The regression test added in this CL is a test that will fail with [0]
without this CL.

[0] https://dart-review.googlesource.com/c/sdk/+/374320

Change-Id: I5184cf6e712f33a894fd1463a5903128f87be92f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/375280
Commit-Queue: Martin Kustermann <kustermann@google.com>
Reviewed-by: Slava Egorov <vegorov@google.com>