CodeGen: TII: Take MachineInstr& in predicate API, NFC

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.

llvm-svn: 261605