Graph construction, graph scheduling, instruction selection, and register
allocation has been moved to ExecuteCompilation, which will eventually be
executed on the background threads. Code generation remains in
FinishCompilation because it has to be executed by the main thread.

Additionally, WasmCompilationUnits are finished more eagerly in
wasm-module.cc to save memory.

R=titzer@chromium.org

Review-Url: https://codereview.chromium.org/1942773002
Cr-Commit-Position: refs/heads/master@{#35973}