Since we only need to store 18 different function kinds,
the bitfield approach was wasting space (requiring 11 bits).

This patch replaces the bitfield with a regular enum, and
updates all the FunctionKind predicates to use comparisons
instead of bitwise ops.

For the small amount of builtin code that depended upon being
able to do masking to determine whether something is a class
constructor, we still store two extra bits on FunctionKind,
which are computed when the SFI is initialized.

If this approach causes performance regressions (i.e., if it
turns out that other code was implicitly depending on masking
for fast checks), we can revert this or address it in
other ways (e.g., by doing similar caching of repeated checks
in the caller).

Change-Id: Iebb3214f564ea8bd7b21e78fda33517d63247124
Reviewed-on: https://chromium-review.googlesource.com/860896
Commit-Queue: Adam Klein <adamk@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50559}