Altivec + 16 byte alignment

[Richard Kenner]

    I think the answer is the minimum of MAX_OFILE_ALIGNMENT, 
    STACK_BOUNDARY and BIGGEST_ALIGNMENT, if there are no bugs.  If there 
    are bugs, you can't, not without fixing those bugs.

Well, then there's another bug.  Look at the usage of REGNO_POINTER_ALIGNMENT
in combine.c: it assumes that the bits not in that alignment are zero.
Then look at all the calls to mark_reg_pointer that just pass TYPE_ALIGN
or DECL_ALIGN but nothing performs the minimum above before setting those.

But I must say that 

    Biggest alignment that any data type can require on this machine, in bits.

still seems in conflict with the current implementation.  If a type can
*require* that alignment, doesn't that mean that we are going to *give*
it that alignment?  But how can we if STACK_BOUNDARY is smaller?

I think this is a real mess at this point.