This extends TryToName by HeapNumber-to-intptr support and cached array
index retrieval from non-internalized strings, and uses it in the
KeyedLoadIC_Generic stub.

Bonus: avoid needless movsxlq on x64 in LoadFixed{,Double}ArrayElement
helpers by introducing INTPTR_PARAMETER mode.

Review-Url: https://codereview.chromium.org/2277363002
Cr-Commit-Position: refs/heads/master@{#39217}