[x86/SLH] Major refactoring of SLH implementaiton. There are two big

changes that are intertwined here:

1) Extracting the tracing of predicate state through the CFG to its own
   function.
2) Creating a struct to manage the predicate state used throughout the
   pass.

Doing brson#1 necessitates and motivates the particular approach for brson#2 as
now the predicate management is spread across different functions
focused on different aspects of it. A number of simplifications then
fell out as a direct consequence.

I went with an Optional to make it more natural to construct the
MachineSSAUpdater object.

This is probably the single largest outstanding refactoring step I have.
Things get a bit more surgical from here. My current goal, beyond
generally making this maintainable long-term, is to implement several
improvements to how we do interprocedural tracking of predicate state.
But I don't want to do that until the predicate state management and
tracing is in reasonably clear state.

Differential Revision: https://reviews.llvm.org/D49427

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@337446 91177308-0d34-0410-b5e6-96231b3b80d8