For LoadElimination we must not replace LoadField nodes with other nodes
whose types are not a subtype of the original LoadField type, as that
breaks the verifier. We already fixed that earlier for store to load
forwarding, but the fix didn't cover LoadField forwarding.

This actually still generates the correct code even w/o the fix, but
since recently fails due to stronger checking in representation
selection. So this makes clusterfuzz happy again.

R=mvstanton@chromium.org
BUG=chromium:607899
LOG=n

Review-Url: https://codereview.chromium.org/1934973002
Cr-Commit-Position: refs/heads/master@{#35930}