The int64-lowering lowers return nodes which return one int64 value into
a return node which returns two int32 values. For this lowering it has
to adjust the input count of the return operator.

The existing code assumed that if the signature of a function said that
the return type is int64, then all return nodes have int64 inputs.
However, with a recent CL we also introduced void returns. With this CL
I check if the number of inputs of a return node changes with the
DefaultLowering, and only if the number of inputs changes, then I check
if I also have to change the operator of the return node.

R=mstarzinger@chromium.org
TEST=mjsunit/regress/wasm/regression-6164
BUG=v8:6164

Change-Id: I004ab1b4be942cc045719f306705d95b48707a1c
Reviewed-on: https://chromium-review.googlesource.com/461941Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44232}