Keyed loads should not unconditionally be compiled to element loads. Update KeyedLoadICs to keep track of the key type, so that Hydrogen can emit ICs for string-keyed loads it doesn't have inline support for.

BUG=v8:3167
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/755513003

Cr-Commit-Position: refs/heads/master@{#25817}