The InstructionSelector::CanCover() heuristic was not correctly set to
match loads that are wired into the effect chain (i.e. when the input
comes from the JavaScript pipeline instead of the RawMachineAssembler).
Also the InstructionSelector on x64 was confused by the
CanBeBetterLeftOperand heuristic, which prevented proper covering for
map checks generated by the JavaScript pipeline.

R=epertoso@chromium.org

Review URL: https://codereview.chromium.org/1734503002

Cr-Commit-Position: refs/heads/master@{#34242}