In the C++ wasm interpreter, we decode LEB encoded immediates each time
we execute the respective instruction. The whole instruction sequence
was validated before, thus we know that all integers are valid.
This CL refactors several Decoder methods to allow for either checked
or unchecked decoding. In the checked case, an error is set if a check
fails, in the unchecked case, a DCHECK will fail.

This improves performance of the interpreter by 20.5%.

R=ahaas@chromium.org
BUG=v8:5822

Change-Id: If69efd4f6fbe19d84bfc2f4aa000f429a8e22bf5
Reviewed-on: https://chromium-review.googlesource.com/468786
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44406}