Since register can be used multiple times on the stack, they can also
be spilled into multiple stack slots. At merge points, we then might
have to reload the spilled slots. A DCHECK currently checks that each
register is only loaded once. Instead of failing, just load the first
stack slot, the others are statically known to contain the same value
anyways.

R=ahaas@chromium.org

Bug: v8:6600, chromium:919533
Change-Id: Ic0c806238b2997f006829b4b509a50468a55befa
Reviewed-on: https://chromium-review.googlesource.com/c/1403124Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58676}