TurboFan expects the offset input of a Load or Store node to be a
pointer-size input, i.e. an int32 input on 32-bit platforms, and int64
on 64-bit platforms. In WebAssembly we always provided 32-bit offset
though, which caused problems when the high word of the register which
contained the offset was not empty.

With this CL we change the offset input to int64 on 64-bit platforms.
In addition we also change the type of the memory_size_ node to int64,
so that that we do not have to adjust the type of the memory size at
every memory load.

This CL will cause performance regressions but is necessary for
correctness and to avoid crashes.

R=titzer@chromium.org

Bug: chromium:766666
Change-Id: I5301e108d05e125258d2a06d500c1b75e91697b8
Reviewed-on: https://chromium-review.googlesource.com/723379Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48689}