Fix DAGCombiner::GatherAllAliases to account for non-chain dependencies

DAGCombiner::GatherAllAliases, which is only used when AA used is enabled
during DAGCombine, had a fundamentally incorrect assumption for which this
change compensates. GatherAllAliases, which is used to find aliasing
predecessor chain nodes (so that a better chain can be selected for a load or
store to enable subsequent optimizations) assumed that walking up the chain
would always catch all possibly-aliasing loads and stores. This is not true: To
really find all aliases, we also need to search for aliases through the value
operand of a store, etc.  Consider the following situation:

  Token1 = ...
  L1 = load Token1, %52
  S1 = store Token1, L1, %51
  L2 = load Token1, %52+8
  S2 = store Token1, L2, %51+8
  Token2 = Token(S1, S2)
  L3 = load Token2, %53
  S3 = store Token2, L3, %52
  L4 = load Token2, %53+8
  S4 = store Token2, L4, %52+8

If we search for aliases of S3 (which loads address %52), and we look only
through the chain, then we'll miss the trivial dependence on L1 (which loads
from %52). We then might change all loads and stores to use Token1 as their
chain operand, which could result in copying %53 into %52 before copying
%52 into %51 (which should happen first).

The problem is, however, that searching for such data dependencies can become
expensive, and the cost is not directly related to the chain depth. Instead,
we'll rule out such configurations by insisting that we've visited all chain
users (except for users of the original chain, which is not necessary).  When
doing this, we need to look through nodes we don't care about (otherwise,
things like register copies will interfere with trivial use cases).

Unfortunately, I don't have a small test case for this problem. Creating the
underlying situation is not hard (a pair of memcpys will do it), but arranging
for the default instruction schedule to be incorrect is very fragile.

This unbreaks self hosting on PPC64 when using
-mllvm -combiner-global-alias-analysis -mllvm -combiner-alias-analysis.