We were looking at the unreachable flag or stack_depth of the target frame
instead of the current one in a couple of places (most notably BreakTo).
This change fixes these bugs and makes us pass the latest spec tests for
br_table validation. Also need to ensure that br_table targets have consistent
types, which is not implied if the stack is polymorphic.

R=titzer@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2696813002
Cr-Commit-Position: refs/heads/master@{#43250}