Port 6689cc27

Original commit message:
Handlers should be in charge of this work. The change uncovered a bug in
vector-ics related to keyed loads into strings. It's important for
StringCharCodeAtGenerator, a helper used in full code and in
LoadIndexedStringStub (a handler) to protect the vector and slot registers
when it makes a runtime call to convert a HeapNumber to a Smi.

It's still possible for the handler to MISS after this call, perhaps due
to out of bounds access. In that case, the vector and slot registers need
to be delivered safely to the MISS handler.

R=mbrandy@us.ibm.com, svenpanne@chromium.org

BUG=

Review URL: https://codereview.chromium.org/1029413002

Cr-Commit-Position: refs/heads/master@{#27430}