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

R=gdeepti@google.com, binji@chromium.org, jyan@ca.ibm.com
BUG=v8:6752
LOG=N

Change-Id: I0633982ff4b5a93551b4765ca8df50073010f3ca
Reviewed-on: https://chromium-review.googlesource.com/633755Reviewed-by: Junliang Yan <jyan@ca.ibm.com>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Jaideep Bajwa <bjaideep@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#47616}