CodeGen: Use MachineInstr& in TargetLowering, NFC

This is a mechanical change to make TargetLowering API take MachineInstr&
(instead of MachineInstr*), since the argument is expected to be a valid
MachineInstr.  In one case, changed a parameter from MachineInstr* to
MachineBasicBlock::iterator, since it was used as an insertion point.

As a side effect, this removes a bunch of MachineInstr* to
MachineBasicBlock::iterator implicit conversions, a necessary step
toward fixing PR26753.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@274287 91177308-0d34-0410-b5e6-96231b3b80d8
36 files changed