[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 #1 necessitates and motivates the particular approach for #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
llvm-svn: 337446