[ARM] Remove static variables from ARMLowOverheadLoops. NFC

VPTState was holding static state, and acting as both the info in a VPTBlock
and the overall state of all the blocks in the loop. This has been split up
into a class (VPTBlock) to hold the instructions of one block, and VPTState
that holds the overall state. The PredicatedInsts is also made into a
map<MachineInstr *, SetVector<MachineInstr *>>, as the double-storing of MI
inside a unique pointer is unneeded.