Reapply r178845 with fix - Fix bug in PEI's virtual-register scavenging

This fixes PEI as previously described, but correctly handles the case where
the instruction defining the virtual register to be scavenged is the first in
the block. Arnold provided me with a bugpoint-reduced test case, but even that
seems too large to use as a regression test. If I'm successful in cleaning it
up then I'll commit that as well.

Original commit message:

    This change fixes a bug that I introduced in r178058. After a register is
    scavenged using one of the available spills slots the instruction defining the
    virtual register needs to be moved to after the spill code. The scavenger has
    already processed the defining instruction so that registers killed by that
    instruction are available for definition in that same instruction. Unfortunately,
    after this, the scavenger needs to iterate through the spill code and then
    visit, again, the instruction that defines the now-scavenged register. In order
    to avoid confusion, the register scavenger needs the ability to 'back up'
    through the spill code so that it can again process the instructions in the
    appropriate order. Prior to this fix, once the scavenger reached the
    just-moved instruction, it would assert if it killed any registers because,
    having already processed the instruction, it believed they were undefined.

    Unfortunately, I don't yet have a small test case. Thanks to Pranav Bhandarkar
    for diagnosing the problem and testing this fix.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178919 91177308-0d34-0410-b5e6-96231b3b80d8
Properly model precise lifetime when given an incomplete dataflow sequence.

The normal dataflow sequence in the ARC optimizer consists of the following
states:

    Retain -> CanRelease -> Use -> Release

The optimizer before this patch stored the uses that determine the lifetime of
the retainable object pointer when it bottom up hits a retain or when top down
it hits a release. This is correct for an imprecise lifetime scenario since what
we are trying to do is remove retains/releases while making sure that no
``CanRelease'' (which is usually a call) deallocates the given pointer before we
get to the ``Use'' (since that would cause a segfault).

If we are considering the precise lifetime scenario though, this is not
correct. In such a situation, we *DO* care about the previous sequence, but
additionally, we wish to track the uses resulting from the following incomplete
sequences:

  Retain -> CanRelease -> Release   (TopDown)
  Retain <- Use <- Release          (BottomUp)

*NOTE* This patch looks large but the most of it consists of updating
test cases. Additionally this fix exposed an additional bug. I removed
the test case that expressed said bug and will recommit it with the fix
in a little bit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178921 91177308-0d34-0410-b5e6-96231b3b80d8
An objc_retain can serve as a use for a different pointer.

This is the counterpart to commit r160637, except it performs the action
in the bottomup portion of the data flow analysis.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178922 91177308-0d34-0410-b5e6-96231b3b80d8