fix inline-asm/15740

[Richard Kenner]

    A, being referencable only from memory, must be marked TREE_ADDRESSABLE.

    It must be done in the front end before gimplification starts, so that
    we don't change a setting of TREE_ADDRESSABLE in the middle, which 
    results in previously correct statements no longer being correct.

What I don't follow is in what way your patch changed that. 
mark_addressable was being called in both the old and new code and
it looks like it was called in very similar circumstances.  That's
the part of the change I don't get.