This CL improves the parsing and validation speed of WASM bytecode by
eliminating a call to get the "simple opcode signature" on the fast
path. It introduces a byte-indexed array that points directly to a
FunctionSig*. By declaring the array in the  wasm-opcodes.h header file
and initializing (constexpr) in the wasm-opcodes.cc file, the decoder
can use this array directly in its first test. (Note that constexpr
obviates the need for LazyInitialize in previous iterations of this
mechanism).

There are two more calls on this fast path that can be simplified,
WasmOpcodes::IsSignExtensionOpcode() and WasmOpcodes::IsAnyRefOpcode().
These calls are needed to check for a feature flag and can be
implemented differently in a followup CL.

R=mstarzinger@chromium.org

Change-Id: Ibb4adb1134932c7e0b6a35facec4d8dd8c998c56
Reviewed-on: https://chromium-review.googlesource.com/1075276Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53391}