[GISel][NFC]: Add some constructors for easy creation of MIRBuilders

[aditya_nandakumar]

Summary

Add some constructors for easier use of MIRBuilders in backend code/passes.
Currently to use the MIRBuilder, we need
MIRBuilder B;
B.setMF(MF);
B.setInstr(SomeMI);
It would be easier to just do
MIRBuilder B(MF, SomeMI);

[qcolombet]

include/llvm/CodeGen/GlobalISel/MachineIRBuilder.h

91

We could have a constructor with only MI, if that makes the usage simpler:
MachineIRBuilder(MI) : MachineIRBuild(MI.getParent()->getParent()) {
setInstr(MI);
}

[aditya_nandakumar]

include/llvm/CodeGen/GlobalISel/MachineIRBuilder.h

91

You mean - instead of the above two constructors, only have one which takes MI?

[qcolombet]

include/llvm/CodeGen/GlobalISel/MachineIRBuilder.h

91

No, I mean one with MF, one with MI instead of the pair (MF MI).
That being said if you prefer (MF, MI) instead of (MI) that works for me.

[aditya_nandakumar]

include/llvm/CodeGen/GlobalISel/MachineIRBuilder.h

91

I see. One with MI should suffice for now.

[aditya_nandakumar]

Updated based on Quentin's feedback.

[qcolombet]

LGTM.
One nit below.

include/llvm/CodeGen/GlobalISel/MachineIRBuilder.h

92

I would rather call the MachineIRBuilder(MF) constructor.

[aditya_nandakumar]

Committed in r310860 with the changes you requested.

include/llvm/CodeGen/GlobalISel/MachineIRBuilder.h

class MachineIRBuilder {
 
   void addUsesFromArgs(MachineInstrBuilder &MIB) { }
   template<typename UseArgTy, typename ... UseArgsTy>
   void addUsesFromArgs(MachineInstrBuilder &MIB, UseArgTy &&Arg1, UseArgsTy &&... Args) {
     addUseFromArg(MIB, Arg1);
     addUsesFromArgs(MIB, std::forward<UseArgsTy>(Args)...);
   }
 public:
+  /// Some constructors for easy use.
+  MachineIRBuilder() = default;
+  MachineIRBuilder(MachineFunction &MF) { setMF(MF); }
+  MachineIRBuilder(MachineFunction &MF, MachineInstr &MI) {
+    setMF(MF);
+    setInstr(MI);
+  }
+
   /// Getter for the function we currently build.
   MachineFunction &getMF() {
     assert(MF && "MachineFunction is not set");
     return *MF;
   }
 
   /// Getter for the basic block we currently build.
   MachineBasicBlock &getMBB() {