During ast decoding and turbofan graph construction, we explicitely pass
the byte offset for all instructions which potentially trap.
The byte offset is finally passed to the runtime function which throws
the actual error, but it is not used there yet.

The WasmGraphBuilder::Binop and Unop methods have a default value of -1
for the position, which allows for more compact code for all the
functions which assemble bigger snippets from the primitive operations.
Whenever the position is actually used for generating a trap, we
check that it is not negative.

R=titzer@chromium.org

Review-Url: https://codereview.chromium.org/1915123006
Cr-Commit-Position: refs/heads/master@{#35925}