Go generic on polymorphic KeyedLoads with string+other maps

The issue is that by handling strings with map/handler pairs instead of a special
version of the keyed load stub (https://code.google.com/p/v8/source/detail?r=24661),
I allowed polymorphism between string and non-string types in the IC. Before, the
IC would go generic.

Then, at crankshaft time, we special case when we only saw strings. The error
here is that crankshaft can't emit code that handles polymorphism between string
and non-string types. The choice is either to get that to happen (I don't deem
this necessary from a performance point of view, an IC with such type feedback
before would have gone generic), or simply check for the case of "polymorphic
with some string maps" and require crankshaft to go generic. I'll do the latter.

BUG=425519
LOG=N
R=jarin@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=24775

[mvstanton, 2014-10-21 12:37:58]

mvstanton@chromium.org changed reviewers:
+ jarin@chromium.org

[mvstanton, 2014-10-21 12:42:43]

Hi Jaroslav, could you have a look? Thanks!
--Michael

[Jarin, 2014-10-21 12:47:47]

lgtm. Thanks!

[mvstanton, 2014-10-21 13:05:01]

Committed patchset #1 (id:1) manually as 24775 (presubmit successful).