Rationale: The {kind} of a call descriptor describes what the {target} being
called is--i.e. a JSFunction, code object, or address. That kind materially
dictates the instruction(s) generated for an outgoing call.

The other flags on a call descriptor should describe specific properties
(like whether a roots register is valid or not) so that backend logic doesn't
have to switch over the kind, but is informed directly of what it wants to
know.

R=mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1268273003

Cr-Commit-Position: refs/heads/master@{#30065}