GlobalISel: Add another overload of buildUnmerge

[arsenm]

Summary

It's annoying to have to create an array of the result type,
particularly when you don't care about the size of the value.

[aditya_nandakumar]

LGTM. Please add a test in MachineIRBuilderTest.cpp if possible.

[arsenm]

r357763

include/llvm/CodeGen/GlobalISel/MachineIRBuilder.h

public:
   /// \pre The entire register \p Res (and no more) must be covered by the input
   ///      registers.
   /// \pre The type of all \p Res registers must be identical.
   ///
   /// \return a MachineInstrBuilder for the newly created instruction.
   MachineInstrBuilder buildUnmerge(ArrayRef<LLT> Res, const SrcOp &Op);
   MachineInstrBuilder buildUnmerge(ArrayRef<unsigned> Res, const SrcOp &Op);
 
+  /// Build and insert an unmerge of \p Res sized pieces to cover \p Op
+  MachineInstrBuilder buildUnmerge(LLT Res, const SrcOp &Op);
+
   /// Build and insert \p Res = G_BUILD_VECTOR \p Op0, ...
   ///
   /// G_BUILD_VECTOR creates a vector value from multiple scalar registers.
   /// \pre setBasicBlock or setMI must have been called.
   /// \pre The entire register \p Res (and no more) must be covered by the
   ///      input scalar registers.
   /// \pre The type of all \p Ops registers must be identical.
   ///

lib/CodeGen/GlobalISel/MachineIRBuilder.cpp

MachineInstrBuilder MachineIRBuilder::buildUnmerge(ArrayRef<LLT> Res,
                                                    const SrcOp &Op) {
   // Unfortunately to convert from ArrayRef<LLT> to ArrayRef<DstOp>,
   // we need some temporary storage for the DstOp objects. Here we use a
   // sufficiently large SmallVector to not go through the heap.
   SmallVector<DstOp, 8> TmpVec(Res.begin(), Res.end());
   return buildInstr(TargetOpcode::G_UNMERGE_VALUES, TmpVec, Op);
 }
 
+MachineInstrBuilder MachineIRBuilder::buildUnmerge(LLT Res,
+                                                   const SrcOp &Op) {
+  unsigned NumReg = Op.getLLTTy(*getMRI()).getSizeInBits() / Res.getSizeInBits();
+  SmallVector<unsigned, 8> TmpVec;
+  for (unsigned I = 0; I != NumReg; ++I)
+    TmpVec.push_back(getMRI()->createGenericVirtualRegister(Res));
+  return buildUnmerge(TmpVec, Op);
+}
+
 MachineInstrBuilder MachineIRBuilder::buildUnmerge(ArrayRef<unsigned> Res,
                                                    const SrcOp &Op) {
   // Unfortunately to convert from ArrayRef<unsigned> to ArrayRef<DstOp>,
   // we need some temporary storage for the DstOp objects. Here we use a
   // sufficiently large SmallVector to not go through the heap.
   SmallVector<DstOp, 8> TmpVec(Res.begin(), Res.end());
   return buildInstr(TargetOpcode::G_UNMERGE_VALUES, TmpVec, Op);
 }