[LLVMdev] Continuing PR5680: preserve order of use lists in bitcode

[Duncan P. N. Exon Smith]

I'm looking to tackle PR5680 [1].  The primary goal is to prevent
behaviour changes in passes that depend on the order of use lists when
serializing/deserializing the IR to a bitcode file (.bc) between passes.

Here's a quick high-level summary:

  - When writing bitcode, calculate what the use list order will be
    after reading the bitcode.  (Assumption: the order is predictable.)

  - Write a use-list-order-diff in a new (optional) section in the
    bitcode.

  - When reading bitcode, apply the use-list-order-diff to restore the
    original use list order.

A secondary goal is to have the same feature for assembly files (.ll).
How to represent the use-list-order(-diff?) in assembly is a little
unclear, so I'll send an RFC when I get to that point.

My plan is to pick up where Chad's commits (e.g., r146078, r146090,
r146442, and r146531) left off.  If someone has a major concern with the
direction, let me know.

[1]: http://llvm.org/bugs/show_bug.cgi?id=5680