Compress the parameter count (and function length) stored in
SharedFunctionInfo to a uint16_t. This limits us to 2^16 - 1 parameters
per function, minus one for the "don't adapt arguments" sentinel value,
which is one fewer than Code::kMaxArguments was already. Anyway, 65534
arguments should be enough for anyone!

This drops SFI size by 4 bytes.

Bug: chromium:818642
Change-Id: I126bfb24453dcdc5087a104d3a12cf195a56fa9f
Reviewed-on: https://chromium-review.googlesource.com/1076627
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53447}