volatile dropped

A common idiom for memory-mapped I/O is this:

```
void f(void)
{
  (*(volatile uint8_t*)42) = 23;
}
```

clang-3.8 correctly compiles this into a volatile store in LLVM IR:

```
define void @f() #0 {
  store volatile i8 23, i8* inttoptr (i64 42 to i8*), align 1
  ret void
}
```

However, the cbe drops the volatile:

```
void f(void) {
  *((uint8_t*)(uintptr_t)UINT64_C(42)) = 23;
}
```

Philipp
