[PBQP] Replace the interference-constraints algorithm with a faster version
loosely based on linear scan.

On x86-64 this is good for a ~2% drop in compile time on the nightly test suite.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220143 91177308-0d34-0410-b5e6-96231b3b80d8