CodeGen: improve MachineInstrBuilder & MachineIRBuilder interface

For MachineInstrBuilder, having to manually use RegState::Define is ugly and
makes register definitions clunkier than they need to be, so this adds two
convenience functions: addDef and addUse.

For MachineIRBuilder, we want to avoid BuildMI's first-reg-is-def rule because
it's hidden away and causes bugs. So this patch switches buildInstr to
returning a MachineInstrBuilder and adding *all* operands via addDef/addUse.

NFC.

llvm-svn: 277176