swift234

Take a big step towards rewriting how definite init promotes loads 
into SSA form, moving to resolving rdar://15170149.

Instead of promoting loads in the middle of the process of proving
that everything passes DI requirements, do this after a full element
is ok.

Doing this requires changing just about everything about how we modeled
elements and subelements being promoted, but this is good for a lot of
reasons:
1) we now don't eagerly scalarize all loads to struct members, now we
   just scalarize top-level tuples.  This is good for preserving higher
   level operations.
2) Even without scalarization, we can still model things at an extremely 
   fine grain, but we can also keep aggregate operations together.
   This means that load promotion doesn't insert a kajillion 
   struct_extract + struct sequences to decompose then recompose things.

This is just patch #1 of a series, but this is the bulk of the hard work.



Swift SVN r9338