port 6689cc27 (r27377)

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.

BUG=

Review URL: https://codereview.chromium.org/1033733005

Cr-Commit-Position: refs/heads/master@{#27461}