Speed up LiveDebugValues

Summary:
Use proper dataflow ordering to speed convergence.
This will converge the testcase on bug 26055 in 2 iterations.

(data structures speedups to come to make even that faster)

Reviewers: kcc, samsonov, echristo, dblaikie, tvvikram

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D16039

llvm-svn: 257292