This matches current Crankshaft/fullcodegen behavior more closely and
thus reduces the chances that we run into unnecessary polymorphism due
to the field representation tracking in our object model.

Drive-by-fixes: Make sure the JSRegExp::lastIndex field stays Smi
if possible (otherwise we tank the regexp benchmark in Octane).

CQ_INCLUDE_TRYBOTS=master.tryserver.v8:v8_mac64_rel,v8_mac64_dbg
R=jarin@chromium.org
BUG=v8:5267

Committed: https://chromium.googlesource.com/v8/v8/+/6a939714e991ebf10d56ddbd2869325cca99c0ef
Committed: https://crrev.com/ee158e6c4cc896479a32245432a3c2fdd31bcb73
Committed: https://crrev.com/ddf792beb3a72f6dba83e94fc8ada03ebf1630bd
Review-Url: https://codereview.chromium.org/2367593003
Cr-Original-Original-Commit-Position: refs/heads/master@{#39692}
Cr-Original-Commit-Position: refs/heads/master@{#39748}
Cr-Commit-Position: refs/heads/master@{#39803}