[MC] Remove MachineInstr reference in MC layer (PR37160)

Only add support for getSchedInfoStr(const MachineInstr &MI) at the TargetSubtargetInfo level.

Really, the getSchedInfoStr calls need to be removed entirely, we should just return a latency/rthroughput through the subtarget and keep a string creation helper function somewhere else.

llvm-svn: 330615