wasm: memset - correctly load the ptr for slices

Previously we would use the address of the slice itself, which would
result in miscompilations and accidently setting the memory region
of the slice itself, rather than based on the `ptr` field.

src/arch/wasm/CodeGen.zig

@@ -4464,7 +4464,9 @@ fn airMemset(func: *CodeGen, inst: Air.Inst.Index, safety: bool) InnerError!void
         .One => @as(WValue, .{ .imm32 = @intCast(u32, ptr_ty.childType().arrayLen()) }),
         .C, .Many => unreachable,
     };
-    try func.memset(ptr, len, value);
+
+    const dst_ptr = try func.sliceOrArrayPtr(ptr, ptr_ty);
+    try func.memset(dst_ptr, len, value);
 
     func.finishAir(inst, .none, &.{ bin_op.lhs, bin_op.rhs });
 }