The int64-lowering only lowered store instructions with a word64 store
representation. For all other stores the default lowering applied. The
default lowering replaces all input nodes with both their replacement
nodes, which can change the number of input nodes of the lowered node.

In WebAssembly there exist stores which take an I64 input and store it
with a different representation, e.g. I32. In TurboFan this translates
to a store node with word32 store representation and a word64 value
input. The default lowering replaces the word64 value input to become
two word32 value inputs, which makes the number of inputs of the store
node invalid. This CL discards the high word replacement of the value
input so that the number of input nodes of a store node does not change
in the default lowering.

R=titzer@chromium.org
CC=rossberg@chromium.org

BUG=

Review-Url: https://codereview.chromium.org/2668023004
Cr-Commit-Position: refs/heads/master@{#42860}