CodeGen: TII: Take MachineInstr& in predicate API, NFC
Change TargetInstrInfo API to take MachineInstr& instead of
MachineInstr* in the functions related to predicated instructions
(I'll try to come back later and get some of the rest). All of these
functions require non-null parameters already, so references are more
clear. As a bonus, this happens to factor away a host of implicit
iterator => pointer conversions.
No functionality change intended.