The existing implementation assumes that return nodes have exactly one
real value input. This assumption does not hold for WebAssembly. To
avoid incorrect behavior, this CL turns of the reduction of returns
with a value input count != 1.

R=titzer@chromium.org, mstarzinger@chromium.org

Review-Url: https://codereview.chromium.org/2638053002
Cr-Commit-Position: refs/heads/master@{#42425}