Some instructions in WebAssembly trap for some inputs, which means that the
execution is terminated and (at least at the moment) a JavaScript exception is
thrown. Examples for traps are out-of-bounds memory accesses, or integer
divisions by zero.

Without the TrapIf and TrapUnless operators trap check in WebAssembly introduces 5
TurboFan nodes (branch, if_true, if_false, trap-reason constant, trap-position
constant), in addition to the trap condition itself. Additionally, each
WebAssembly function has four TurboFan nodes (merge, effect_phi, 2 phis) whose
number of inputs is linear to the number of trap checks in the function.
Especially for functions with high numbers of trap checks we observe a
significant slowdown in compilation time, down to 0.22 MiB/s in the sqlite
benchmark instead of the average of 3 MiB/s in other benchmarks. By introducing
a TrapIf common operator only a single node is necessary per trap check, in
addition to the trap condition. Also the nodes which are shared between trap
checks (merge, effect_phi, 2 phis) would disappear. First measurements suggest a
speedup of 30-50% on average.

This CL only implements TrapIf and TrapUnless on x64. The implementation is also
hidden behind the --wasm-trap-if flag.

Please take a special look at how the source position is transfered from the
instruction selector to the code generator, and at the context that is used for
the runtime call.

R=titzer@chromium.org

Review-Url: https://codereview.chromium.org/2562393002
Cr-Commit-Position: refs/heads/master@{#41720}