Using uint32 to store the the number of control outputs allows WebAssembly switches to have more than 2^16 case.

BUG=v8:5531
TEST=mjsunit/regress/wasm/regression-5531
R=titzer@chromium.org

Review-Url: https://chromiumcodereview.appspot.com/2425983002
Cr-Commit-Position: refs/heads/master@{#40420}