WasmGraphBuilder::StoreMem is called with the last argument
as default with machine rep = kNode, which causes
BuildChangeEndiannessStore(val, memtype, type) to fail.

Originally added this fix in https://chromium-review.googlesource.com/633755
which got removed in this refactor https://chromium-review.googlesource.com/640591

R=clemensh@chromium.org, titzer@chromium.org, binji@chromium.org, jyan@ca.ibm.com
BUG=
LOG=N

Change-Id: I441e9b67517a31df780b35d2403c688d667b6789
Reviewed-on: https://chromium-review.googlesource.com/661097Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Junliang Yan <jyan@ca.ibm.com>
Commit-Queue: Jaideep Bajwa <bjaideep@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#47984}