VectorICs: megamorphic keyed loads in crankshaft don't need a vector.

VectorICs: megamorphic keyed loads in crankshaft don't need a vector.

They are content with a dummy vector, as MISSES won't result in
changing the real vector/slot at all.

BUG=

Review URL: https://codereview.chromium.org/1067573003

Cr-Commit-Position: refs/heads/master@{#27788}