mem-ssa merge 1/3 and 2/3

[Diego Novillo]

I finally yielded to Zdenek's pressure to split the patch.  The split 
makes no logical sense, but they are three separate chunks that I could 
create with the least aggravation.

I didn't make it clear enough that all of mem-ssa evolved in layers: 
VOPS changes -> push/pop -> V_MUST_DEF removal -> partitions -> alias 
speedups.  Separating the implementation post-mortem is fairly annoying.

Tested on x86-64.  Will test and commit the final patch on all the usual 
arches and provide the final numbers shortly.
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