This fixes behavior for HeapNumber {index} arguments passed to
AdvanceStringIndex.

Previously, we'd blindly treat {index} as a Smi. Passing a HeapNumber instead
would result in a Smi addition on the tagged HeapNumber pointer.

BUG=chromium:709015

Review-Url: https://codereview.chromium.org/2798933003
Cr-Commit-Position: refs/heads/master@{#44458}