We hit a DCHECK in the wasm graph builder because the current SSA
environment is unreachable. We were using the wrong block (the target
block) to do the reachability check.

R=titzer@chromium.org

Bug: chromium:913804
Change-Id: I4cfd3a0c696fb63903a47e4448362626a524340d
Reviewed-on: https://chromium-review.googlesource.com/c/1371566Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58150}