[DDG] Data Dependence Graph - Ordinals

Summary:
This patch associates ordinal numbers to the DDG Nodes allowing
the builder to order nodes within a pi-block in program order. The
algorithm works by simply assuming the order in which the BBList
is fed into the builder. The builder already relies on the blocks being
in program order so that it can compute the dependencies correctly.
Similarly the order of instructions in their parent basic blocks
determine their program order.

Authored By: bmahjour

Reviewer: Meinersbur, fhahn, myhsu, xtian, dmgreen, kbarton, jdoerfert

Reviewed By: Meinersbur

Subscribers: ychen, arphaman, simoll, a.elovikov, mgorny, hiraditya, jfb, wuzish, llvm-commits, jsji, Whitney, etiotto, ppc-slack

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70986