The scheduler expects a trimmed graph, so we have to trim the graph
before scheduling.

R=titzer@chromium.org, bmeurer@chromium.org
TEST=cctest/test-run-wasm/RunWasmCompiled_GraphTrimming

Review-Url: https://chromiumcodereview.appspot.com/2428443002
Cr-Commit-Position: refs/heads/master@{#40446}