Hydrogen: fix keyed loads with string keys

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}