[RewriteStatepointsForGC] Make base pointer inference deterministic

Previously, the base pointer algorithm wasn't deterministic. The core fixed point was (of course), but we were inserting new nodes and optimizing them in an order which was unspecified and variable. We'd somewhat hacked around this for testing by sorting by value name, but that doesn't solve the general determinism problem.

Instead, we can use the order of traversal over the def/use graph to give us a single consistent ordering. Today, this is a DFS order, but the exact order doesn't mater provided it's deterministic for a given input.

(Q: It is safe to rely on a deterministic order of operands right?)

Note that this only fixes the determinism within a single inference step. The inference step is currently invoked many times in a non-deterministic order. That's a future change in the sequence. :)

Differential Revision: http://reviews.llvm.org/D12640

llvm-svn: 247208