[wasm fuzzer] fix i64.store generation error

This was meant to be included in https://crrev.com/c/757129 but got missed
somehow. The fuzzer was generating i64.store instructions with an i32 value
argument instead of i64 like it should be.

Bug: 
Change-Id: I5b5bcdb22b2ac3abe872e7ff0ab0019b5ecb9c98
Reviewed-on: https://chromium-review.googlesource.com/759148Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Eric Holk <eholk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49270}

test/fuzzer/wasm-compile.cc

@@ -229,7 +229,7 @@ void WasmGenerator::Generate<kWasmStmt>(DataRange data) {
       &WasmGenerator::memop<kExprI32StoreMem, kWasmI32>,
       &WasmGenerator::memop<kExprI32StoreMem8, kWasmI32>,
       &WasmGenerator::memop<kExprI32StoreMem16, kWasmI32>,
-      &WasmGenerator::memop<kExprI64StoreMem, kWasmI32>,
+      &WasmGenerator::memop<kExprI64StoreMem, kWasmI64>,
       &WasmGenerator::memop<kExprI64StoreMem8, kWasmI64>,
       &WasmGenerator::memop<kExprI64StoreMem16, kWasmI64>,
       &WasmGenerator::memop<kExprI64StoreMem32, kWasmI64>,