[asan] Emit GIMPLE directly, small cleanups

[Wei Mi]

Hi Diego,

>>     /* Build
>> -     (base_addr >> ASAN_SHADOW_SHIFT) | targetm.asan_shadow_offset ().
>> */
>> +     (base_addr >> ASAN_SHADOW_SHIFT) + targetm.asan_shadow_offset ().
>> */
>
>
> Hm, I wonder if this is a documentation problem or we're generating bad
> runtime code.  Wei, you tested the runtime and it was working with the GCC
> generated code, right?
>
> In any case, we can adjust the expression later.

I only tested my smallcase and it worked. Because usually the redzone
are not a very small areas (more than 4K), so I think it is possible
the smallcase works even if the shadow addr calculation is incorrect
and has small deviation.

Thanks,
Wei.