This CL refactors the implementation of WASM->JS import wrappers in order
to make the wrapper code shareable. Instead of specializing to the import
index, we use a tuple as the object ref in the both the import and indirect
tables. The tuple allows the wrapper code to load both the calling
instance and the target callable, rather than relying on code specialization.

This requires some tricky codegen machinery, because WASM call descriptors
expect an instance argument in a given register, yet the wrappers receive
a tuple, the code generator must generate a prologue that loads the
instance (and the callable), since it is not possible to express this at
the graph level.

R=mstarzinger@chromium.org
CC=clemensh@chromium.org

Change-Id: Id67e307f7f5089e776f5439a53b5aee4b76934b6
Reviewed-on: https://chromium-review.googlesource.com/c/1268237
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56520}