s390: [wasm][cleanup] Return void in some WasmGraphBuilder public functions

Port 7e6fe4ea

Original Commit Message:

    Although the result was unused, these functions used to return a (often
    random) Node* to satisfy old restrictions of graph-builder-interface.
    Now that these restrictions are lifted, we can type them properly as
    {void}.

R=manoskouk@chromium.org, joransiu@ca.ibm.com, junyan@redhat.com, midawson@redhat.com
BUG=
LOG=N

Change-Id: Iabcc889af26ddb2325530dc78d15f5a8f4667387
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2775570
Commit-Queue: Milad Fa <mfarazma@redhat.com>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73547}

src/compiler/wasm-compiler.cc

@@ -4189,7 +4189,6 @@ void WasmGraphBuilder::StoreLane(MachineRepresentation mem_rep, Node* index,
                                  uint8_t laneidx,
                                  wasm::WasmCodePosition position,
                                  wasm::ValueType type) {
-  Node* store;
   has_simd_ = true;
   index = BoundsCheckMem(i::ElementSizeInBytes(mem_rep), index, offset,
                          position, kCanOmitBoundsCheck);
@@ -4213,13 +4212,13 @@ void WasmGraphBuilder::StoreLane(MachineRepresentation mem_rep, Node* index,
   } else {
     UNREACHABLE();
   }
-  store = StoreMem(mem_rep, index, offset, alignment, output, position, type);
+  StoreMem(mem_rep, index, offset, alignment, output, position, type);
 #else
   MachineType memtype = MachineType(mem_rep, MachineSemantic::kNone);
   MemoryAccessKind load_kind =
       GetMemoryAccessKind(mcgraph(), memtype, use_trap_handler());
 
-  store = SetEffect(graph()->NewNode(
+  Node* store = SetEffect(graph()->NewNode(
       mcgraph()->machine()->StoreLane(load_kind, mem_rep, laneidx),
       MemBuffer(capped_offset), index, val, effect(), control()));